Added anchors and references for link sharing policies.

[simgrid.git] / doc / doxygen / platform.doc

/*! \page platform Platform Description

@tableofcontents

In order to run any simulation, SimGrid must be provided with three things:
something to run (i.e., your code), a description of the platform on which you
want to simulate your application and lastly information about the deployment
process: Which process should be deployed to which processor/core?

For the last two items, there are essentially two possible ways you can provide
this information as an input:
\li You can program it, either by using the Lua console (
    \ref MSG_Lua_funct) or, if you're using MSG, some of MSG's platform and
    deployment functions (\ref msg_simulation). If you want to use this,
    check the particular documentation. (You can also check the section
    \ref pf_flexml_bypassing, however, this documentation is deprecated;
    there is a new, but undocumented, way to do it properly).
\li You can use two XML files: one contains the platform description while
    the other contains the deployment instructions.

For more information on SimGrid's deployment features, please refer to
the \ref deployment documentation.

The platform description may be intricate. This documentation is all
about how to write this file: The basic concepts are introduced. Furthermore,
advanced options are explained. Additionally, some hints and tips on how to
write a good platform description are given.

\section pf_overview Some words about XML and DTD

We chose to use XML not only because it's extensible but also because many
tools (and plugins for existing tools) are available that facilitate editing and
validating XML files. Furthermore, libraries that parse XML are often already
available and very well tested.

The XML checking is done based on the Document Type Definition (DTD) file,
available at
<a href="http://simgrid.gforge.inria.fr/simgrid.dtd">http://simgrid.gforge.inria.fr/simgrid.dtd</a>.

If you read the DTD, you should notice the following:
\li The platform tags contain a version attribute; the current version is 3.
    This property might be used in the future to provide backwards
    compatibility.
\li The DTD contains definitions for the two files used by SimGrid (i.e.,
    platform description and deployment).

\section pf_basics Basic concepts

Nowadays, the Internet is composed of a bunch of independently managed
networks. Within each of those networks, there are entry and exit
points (most of the time, you can both enter and exit through the same
point); this allows to leave the current network and reach other
networks, possibly even in other locations.
At the upper level, such a network is called
<b>Autonomous System (AS)</b>, while at the lower level it is named
sub-network, or LAN (local area network).
They are indeed autonomous: routing is defined
(within the limits of his network) by the administrator, and so, those
networks can operate without a connection to other
networks. So-called gateways allow you to go from one network to
another, if such a (physical) connection exists. Every node in one network
that can be directly reached (i.e., without traversing other nodes) from
another network is called a gateway.
Each autonomous system consists of equipment such as cables (network links),
routers and switches as well as computers.

The structure of the SimGrid platform description relies exactly on the same
concept as a real-life platform (see above).  Every resource (computers,
network equipment etc.) belongs to an AS, which can be defined by using the
\<AS\> tag. Within an AS, the routing between its elements can be defined
abitrarily. There are several modes for routing, and exactly one mode must be
selected by specifying the routing attribute in the AS tag:

\verbatim
<AS id="AS0" routing="Full">
\endverbatim

\remark
  Other supported values for the routing attribute can be found below, Section
  \ref pf_raf.

There is also the ``<route>`` tag; this tag takes two attributes, ``src`` (source)
and ``dst`` (destination). Both source and destination must be valid identifiers
for routers (these will be introduced later). Contained by the ``<route>`` are
network links; these links must be used in order to communicate from the source
to the destination specified in the tag. Hence, a route merely describes
how to reach a router from another router.

\remark
  More information and (code-)examples can be found in Section \ref pf_rm.

An AS can also contain itself one or more AS; this allows you to
model the hierarchy of your platform.

### Within each AS, the following types of resources exist:

%Resource        | Documented in Section | Description
--------------- | --------------------- | -----------
AS              |                       | Every Autonomous System (AS) may contain one or more AS.
host            | \ref pf_host          | This entity carries out the actual computation. For this reason, it contains processors (with potentially multiple cores).
router          | \ref pf_router        | In SimGrid, routers are used to provide helpful information to routing algorithms.  Routers may also act as gateways, connecting several autonomous systems with each other.
link            | \ref pf_link          | In SimGrid, (network)links define a connection between two or potentially even more resources. Every link has a bandwidth and a latency and may potentially experience congestion.
cluster         | \ref pf_cluster       | In SimGrid, clusters were introduced to model large and homogenous environments. They are not really a resource by themselves - technically, they are only a shortcut, as they will internally set up all the hosts, network and routing for you, i.e., using this resource, one can easily setup thousands of hosts and links in a few lines of code. Each cluster is itself an AS.

%As it is desirable to interconnect these resources, a routing has to be
defined. The AS is supposed to be Autonomous, hence this has to be done at the
AS level. The AS handles two different types of entities (<b>host/router</b>
and <b>AS</b>). However, the user is responsible to define routes between those resources,
otherwise entities will be unconnected and therefore unreachable from other
entities. Although several routing algorithms are built into SimGrid (see
\ref pf_rm), you might encounter a case where you want to define routes
manually (for instance, due to specific requirements of your platform).

There are three tags to use:
\li <b>ASroute</b>: to define routes between two  <b>AS</b>
\li <b>route</b>: to define routes between two <b>host/router</b>
\li <b>bypassRoute</b>: to define routes between two <b>AS</b> that
    will bypass default routing (as specified by the ``routing`` attribute
    supplied to ``<AS>``, see above).

Here is an illustration of these concepts:

![An illustration of an AS hierarchy. Here, AS1 contains 5 other AS' who in turn may contain other AS' as well.](AS_hierarchy.png)
 Circles represent processing units and squares represent network routers. Bold
    lines represent communication links. AS2 models the core of a national
    network interconnecting a small flat cluster (AS4) and a larger
    hierarchical cluster (AS5), a subset of a LAN (AS6), and a set of peers
    scattered around the world (AS7).

\section pf_pftags Resource description

\subsection  pf_As Platform: The \<AS\> tag

The concept of an AS was already outlined above (Section \ref pf_basics);
recall that the AS is so important because it groups other resources (such
as routers/hosts) together (in fact, these resources must be contained by
an AS).

Available attributes :

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | String | The identifier of an AS; facilitates referring to this AS. ID must be unique.
routing         | yes       | Full\| Floyd\| Dijkstra\| DijkstraCache\| none\| Vivaldi\| Cluster | See Section \ref pf_rm for details.


<b>Example:</b>
\verbatim
<AS id="AS0" routing="Full">
   <host id="host1" power="1000000000"/>
   <host id="host2" power="1000000000"/>
   <link id="link1" bandwidth="125000000" latency="0.000100"/>
   <route src="host1" dst="host2"><link_ctn id="link1"/></route>
</AS>
\endverbatim

In this example, AS0 contains two hosts (host1 and host2). The route
between the hosts goes through link1.

\subsection pf_Cr Computing resources: hosts, clusters and peers.

\subsubsection pf_host The tag <host/>

A <b>host</b> represents a computer/node card. Every host is able to execute
code and it can send and receive data to/from other hosts. Most importantly,
a host can contain more than 1 core.

### Attributes: ###

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | String | The identifier of the host. facilitates referring to this AS.
power           | yes       | double (must be > 0.0) | Computational power of every core of this host in FLOPS. Must be larger than 0.0.
core            | no        | int (Default: 1) | The number of cores of this host. If more than one core is specified, the "power" parameter refers to every core, i.e., the total computational power is #cores*power.<br /> If 6 cores are specified, up to 6 tasks can be executed without sharing the computational power; if more than 6 tasks are executed, computational power will be shared among these tasks. <br /> <b>Warning:</b> Although functional, this model was never scientifically assessed.
availability    | no        | int    | <b>Specify if the percentage of power available.</b> (What? TODO)
availability_file| no       | string | (Relative or absolute) filename to use as input; must contain availability traces for this host. The syntax of this file is defined below. <br /> <b>Note:</b> The filename must be specified with your system's format.
state           | no        | ON\|OFF<br/> (Default: ON) | Is this host running or not?
state_file      | no        | string |  Same mechanism as availability_file.<br /> <b>Note:</b> The filename must be specified with your system's format.
coordinates     | no        | string | Must be provided when choosing the Vivaldi, coordinate-based routing model for the AS the host belongs to. More details can be found in the Section \ref pf_P2P_tags.

### Possible children: ###

Tag name        | Description | Documentation
------------    | ----------- | -------------
<mount/>        | Defines mounting points between some storage resource and the host. | \ref pf_sto_mo
<prop/>         | The prop tag allows you to define additional information on this host following the attribute/value schema. You may want to use it to give information to the tool you use for rendering your simulation, for example. | N/A

### Example ###

\verbatim
   <host id="host1" power="1000000000"/>
   <host id="host2" power="1000000000">
        <prop id="color" value="blue"/>
        <prop id="rendershape" value="square"/>
   </host>
\endverbatim


\anchor pf_host_dynamism
### Expressing dynamism ###

SimGrid provides mechanisms to change a hosts' availability over
time, using the ``availability_file`` attribute to the ``<host>`` tag
and a separate text file whose syntax is exemplified below.

#### Adding a trace file ####

\verbatim
<platform version="1">
  <host id="bob" power="500000000" availability_file="bob.trace" />
</platform>
\endverbatim

#### Example of "bob.trace" file ####

~~~~~~~~~~~~~~{.py}
PERIODICITY 1.0
  0.0 1.0
  11.0 0.5
  20.0 0.8
~~~~~~~~~~~~~~

Let us begin to explain this example by looking at line 2. (Line 1 will become clear soon).
The first column describes points in time, in this case, time 0. The second column
describes the relative amount of power this host is able to deliver (relative
to the maximum performance specified in the ``<host>`` tag). (Clearly, the
second column needs to contain values that are not smaller than 0 and not larger than 1).
In this example, our host will deliver 500 Mflop/s at time 0, as 500 Mflop/s is the
maximum performance of this host. At time 11.0, it will
deliver half of its maximum performance, i.e., 250 Mflop/s until time 20.0 when it will
will start delivering 80\% of its power. In this example, this amounts to 400 Mflop/s.

Since the periodicity in line 1 was set to be 1.0, i.e., 1 timestep, this host will
continue to provide 500 Mflop/s from time 21. From time 32 it will provide 250 MFlop/s and so on.

### Changing initial state ###

It is also possible to specify whether the host is up or down by setting the
``state`` attribute to either <b>ON</b> (default value) or <b>OFF</b>.

#### Example: Expliciting the default value "ON" ####

\verbatim
<platform version="1">
   <host id="bob" power="500000000" state="ON" />
</platform>
\endverbatim

If you want this host to be unavailable, simply substitute ON with OFF.

### Expressing churn ###

To express the fact that a host can change state over time (as in P2P
systems, for instance), it is possible to use a file describing the time
at which the host is turned on or off. An example of the content
of such a file is presented below.

#### Adding a state file ####

\verbatim
<platform version="1">
  <host id="bob" power="500000000" state_file="bob.fail" />
</platform>
\endverbatim

#### Example of "bob.fail" file ####

~~~{.py}
  PERIODICITY 10.0
  1.0 -1.0
  2.0 1.0
~~~

A negative value means <b>down</b> (i.e., OFF) while a positive one means <b>up and
  running</b> (i.e., ON). From time 0.0 to time 1.0, the host is on. At time 1.0, it is
turned off and at time 2.0, it is turned on again until time 12 (2.0 plus the
periodicity 10.0). It will be turned on again at time 13.0 until time 23.0, and
so on.



\subsubsection pf_cluster <cluster>

``<cluster />`` represents a machine-cluster. It is most commonly used
when one wants to define many hosts and a network quickly. Technically,
``cluster`` is a meta-tag: <b>from the inner SimGrid point of
view, a cluster is an AS where some optimized routing is defined</b>.
The default inner organization of the cluster is as follow:

\verbatim
                 __________
                |          |
                |  router  |
    ____________|__________|_____________ backbone
      |   |   |              |     |   |
    l0| l1| l2|           l97| l96 |   | l99
      |   |   |   ........   |     |   |
      |                                |
    c-0.me                             c-99.me
\endverbatim

Here, a set of <b>host</b>s is defined. Each of them has a <b>link</b>
to a central backbone (backbone is a link itself, as a link can
be used to represent a switch, see the switch / link section
below for more details about it). A <b>router</b> allows to connect a
<b>cluster</b> to the outside world. Internally,
SimGrid treats a cluster as an AS containing all hosts: the router is the default
gateway for the cluster.

There is an alternative organization, which is as follows:
\verbatim
                 __________
                |          |
                |  router  |
                |__________|
                    / | \
                   /  |  \
               l0 / l1|   \l2
                 /    |    \
                /     |     \
            host0   host1   host2
\endverbatim

The principle is the same, except that there is no backbone. This representation
can be obtained easily: just do not set the bb_* attributes.


Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the cluster. Facilitates referring to this cluster.
prefix          | yes       | string | Each node of the cluster has to have a name. This name will be prefixed with this prefix.
suffix          | yes       | string | Each node of the cluster will be suffixed with this suffix
radical         | yes       | string | Regexp used to generate cluster nodes name. Syntax: "10-20" will give you 11 machines numbered from 10 to 20, "10-20;2" will give you 12 machines, one with the number 2, others numbered as before. The produced number is concatenated between prefix and suffix to form machine names.
power           | yes       | int    | Same as the ``power`` attribute of the ``<host>`` tag.
core            | no        | int (default: 1) | Same as the ``core`` attribute of the ``<host>`` tag.
bw              | yes       | int    | Bandwidth for the links between nodes and backbone (if any). <b>See <b>link</b> section for syntax/details.</b>
lat             | yes       | int    | Latency for the links between nodes and backbone (if any). See <b>link</b> section for syntax/details.
sharing_policy  | no        | string | Sharing policy for the links between nodes and backbone (if any). See <b>link</b> section for syntax/details.
bb_bw           | no        | int    | Bandwidth for backbone (if any). See <b>link</b> section for syntax/details. If bb_bw and bb_lat (see below) attributes are omitted, no backbone is created (alternative cluster architecture <b>described before</b>).
bb_lat          | no        | int    | Latency for backbone (if any). See <b>link</b> section for syntax/details. If bb_lat and bb_bw (see above) attributes are omitted, no backbone is created (alternative cluster architecture <b>described before</b>).
bb_sharing_policy | no      | string | Sharing policy for the backbone (if any). See <b>link</b> section for syntax/details.
availability_file | no      | string | Allows you to use a file as input for availability. Similar to <b>hosts</b> attribute.
state_file        | no      | string | Allows you to use a file as input for states.  Similar to <b>hosts</b> attribute.
loopback_bw       | no      | int    | Bandwidth for loopback (if any). See <b>link</b> section for syntax/details. If loopback_bw and loopback_lat (see below) attributes are omitted, no loopback link is created and all intra-node communication will use the main network link of the node. Loopback link is a \ref sharing_policy_fatpipe "\b FATPIPE".
loopback_lat      | no      | int    | Latency for loopback (if any). See <b>link</b> section for syntax/details. See loopback_bw for more info.
topology          | no      | FLAT\|TORUS\|FAT_TREE (default: FLAT) | Network topology to use. SimGrid currently supports FLAT (with or without backbone, as described before), <a href="http://en.wikipedia.org/wiki/Torus_interconnect">TORUS </a> and FAT_TREE attributes for this tag.
topo_parameters   | no      | string | Specific parameters to pass for the topology defined in the topology tag. For torus networks, comma-separated list of the number of nodes in each dimension of the torus. For fat trees, refer to \ref AsClusterFatTree "AsClusterFatTree documentation".

TODO

the router name is defined as the resulting String in the following
java line of code:

@verbatim
router_name = prefix + clusterId + _router + suffix;
@endverbatim


#### Cluster example ####

Consider the following two (and independent) uses of the ``cluster`` tag:

\verbatim
<cluster id="my_cluster_1" prefix="" suffix="" radical="0-262144"
         power="1e9" bw="125e6" lat="5E-5"/>

<cluster id="my_cluster_2" prefix="c-" suffix=".me" radical="0-99"
         power="1e9" bw="125e6" lat="5E-5"
         bb_bw="2.25e9" bb_lat="5E-4"/>
\endverbatim

The second example creates one router and 100 machines with the following names:
\verbatim
c-my_cluster_2_router.me
c-0.me
c-1.me
c-2.me
...
c-99.me
\endverbatim

\subsubsection pf_peer <peer/>

This tag represents a peer, as in Peer-to-Peer (P2P) networks. However, internally,
SimGrid transforms a peer into an AS (similar to Cluster). Hence, this tag
is virtually only a shortcut that comes with some pre-defined resources
and values. These are:

\li A tiny AS whose routing type is cluster is created
\li A host
\li Two links: One for download and one for upload. This is
    convenient to use and simulate stuff under the last mile model (e.g., ADSL peers).
\li It connects the two links to the host
\li It creates a router (a gateway) that serves as an entry point for this peer zone.
    This router has coordinates.

#### Attributes ####

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the peer. Facilitates referring to this peer.
power           | yes       | int    | See the description of the ``host`` tag for this attribute
bw_in           | yes       | int    | Bandwidth downstream
bw_out          | yes       | int    | Bandwidth upstream
lat             | yes       | double | Latency for both up- and downstream, in seconds.
coordinates     | no        | string | Coordinates of the gateway for this peer. Example value: 12.8 14.4 6.4
sharing_policy  | no        | SHARED\|FULLDUPLEX (default: FULLDUPLEX) | Sharing policy for links. See <b>link</b> description for details.
availability_file| no       | string | Availability file for the peer. Same as host availability file. See <b>host</b> description for details.
state_file      | no        | string | State file for the peer. Same as host state file. See <b>host</b> description for details.

Internally, SimGrid transforms any ``<peer/>`` construct such as
\verbatim
<peer id="FOO"
  coordinates="12.8 14.4 6.4"
  power="1.5Gf"
  bw_in="2.25GBps"
  bw_out="2.25GBps"
  lat="500us" />
\endverbatim
into an ``<AS>`` (see Sections \ref pf_basics and \ref pf_As). In fact, this example of the ``<peer/>`` tag
is completely equivalent to the following declaration:

\verbatim
<AS id="as_FOO" routing="Cluster">
   <host id="peer_FOO" power="1.5Gf"/>
   <link id="link_FOO_UP" bandwidth="2.25GBps" latency="500us"/>
   <link id="link_FOO_DOWN" bandwidth="2.25GBps" latency="500us"/>
   <router id="router_FOO" coordinates="25.5 9.4 1.4"/>
   <host_link id="peer_FOO" up="link_FOO_UP" down="link_FOO_DOWN"/>
</AS>
\endverbatim


\subsection pf_ne Network equipments: links and routers

There are two tags at all times available to represent network entities and
several other tags that are available only in certain contexts.
1. ``<link>``: Represents a entity that has a limited bandwidth, a
    latency, and that can be shared according to TCP way to share this
    bandwidth.
\remark
  The concept of links in SimGrid may not be intuitive, as links are not limited
  to connecting (exactly) two entities; in fact, you can have more than two equipments
  connected to it. (In graph theoretical terms: A link in SimGrid is not an edge,
  but a hyperedge)

2. ``<router/>``: Represents an entity that a message can be routed
    to, but that is unable to execute any code. In SimGrid, routers have also
    no impact on the performance: Routers do not limit any bandwidth nor
    do they increase latency. As a matter of fact, routers are (almost) ignored
    by the simulator when the simulation has begun.

3. ``<backbone/>``: This tag is only available when the containing AS is
                    used as a cluster (i.e., mode="Cluster")

\remark
    If you want to represent an entity like a switch, you must use ``<link>`` (see section). Routers are used
    to run some routing algorithm and determine routes (see Section \ref pf_routing for details).

\subsubsection pf_router <router/>

%As said before, <b>router</b> is used only to give some information
for routing algorithms. So, it does not have any attributes except :

#### Attributes ####

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the router to be used when referring to it.
coordinates     | yes       | string | Must be provided when choosing the Vivaldi, coordinate-based routing model for the AS the router belongs to. More details can be found in the Section \ref pf_P2P_tags.

#### Example ####

\verbatim
 <router id="gw_dc1_horizdist"/>
\endverbatim

\subsubsection pf_link <link>

Network links can represent one-hop network connections. They are
characterized by their id and their bandwidth; links can (but may not) be subject
to latency.

#### Attributes ####

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the link to be used when referring to it.
bandwidth       | yes       | int    | Maximum bandwidth for this link, given in bytes/s
latency         | no        | double (default: 0.0) | Latency for this link.
sharing_policy  | no        | \ref sharing_policy_shared "SHARED"\|\ref sharing_policy_fatpipe "FATPIPE"\|\ref sharing_policy_fullduplex "FULLDUPLEX" (default: SHARED) | Sharing policy for the link.
state           | no        | ON\|OFF (default: ON) | Allows you to to turn this link on or off (working / not working)
bandwidth_file  | no        | string | Allows you to use a file as input for bandwidth.
latency_file    | no        | string | Allows you to use a file as input for latency.
state_file      | no        | string | Allows you to use a file as input for states.


#### Possible shortcuts for ``latency`` ####

When using the latency attribute, you can specify the latency by using the scientific
notation or by using common abbreviations. For instance, the following three tags
are equivalent:

\verbatim
 <link id="LINK1" bandwidth="125000000" latency="5E-6"/>
 <link id="LINK1" bandwidth="125000000" latency="5us"/>
 <link id="LINK1" bandwidth="125000000" latency="0.000005"/>
\endverbatim

Here, the second tag uses "us", meaning "microseconds". Other shortcuts are:

Name | Abbreviation | Time (in seconds)
---- | ------------ | -----------------
Week | w | 7 * 24 * 60 * 60
Day  | d | 24 * 60 * 60
Hour | h | 60 * 60
Minute | m | 60
Second | s | 1
Millisecond | ms | 0.001 = 10^(-3)
Microsecond | us | 0.000001 = 10^(-6)
Nanosecond  | ns | 0.000000001 = 10^(-9)
Picosecond  | ps | 0.000000000001 = 10^(-12)

#### Sharing policy ####

\anchor sharing_policy_shared
By default a network link is \b SHARED, i.e., if two or more data flows go
through a link, the bandwidth is shared fairly among all data flows. This
is similar to the sharing policy TCP uses.

\anchor sharing_policy_fatpipe
On the other hand, if a link is defined as a \b FATPIPE,
each flow going through this link will be provided with the complete bandwidth,
i.e., no sharing occurs and the bandwidth is only limiting each flow individually.
The complete bandwidth provided by this link in this mode
is ``#flows*bandwidth``, with at most ``bandwidth`` being available per flow.

Using the FATPIPE mode allows to model backbones that won't affect performance
(except latency).

\anchor sharing_policy_fullduplex
The last mode available is \b FULLDUPLEX. This means that SimGrid will
automatically generate two links (one carrying the suffix _UP and the other the
suffix _DOWN) for each ``<link>`` tag. This models situations when the direction
of traffic is important.

\remark
  Transfers from one side to the other will interact similarly as
  TCP when ACK returning packets circulate on the other direction. More
  discussion about it is available in the description of link_ctn description.

In other words: The SHARED policy defines a physical limit for the bandwidth.
The FATPIPE mode defines a limit for each application,
with no upper total limit.

\remark
  Tip: By using the FATPIPE mode, you can model big backbones that
  won't affect performance (except latency).

#### Example ####

\verbatim
 <link id="SWITCH" bandwidth="125000000" latency="5E-5" sharing_policy="FATPIPE" />
\endverbatim

#### Expressing dynamism and failures ####

Similar to hosts, it is possible to declare links whose state, bandwidth
or latency changes over time (see Section \ref pf_hosts_dynamism for details).

In the case of network links, the ``bandwidth`` and ``latency`` attributes are
replaced by the ``bandwidth_file`` and ``latency_file`` attributes.
The following XML snippet demonstrates how to use this feature in the platform
file. The structure of the files "link1.bw" and "link1.lat" is shown below.

\verbatim
<link id="LINK1" state_file="link1.fail" bandwidth="80000000" latency=".0001" bandwidth_file="link1.bw" latency_file="link1.lat" />
\endverbatim

\note
  Even if the syntax is the same, the semantic of bandwidth and latency
  trace files differs from that of host availability files. For bandwidth and
  latency, the corresponding files do not
  express availability as a fraction of the available capacity but directly in
  bytes per seconds for the bandwidth and in seconds for the latency. This is
  because most tools allowing to capture traces on real platforms (such as NWS)
  express their results this way.

##### Example of "link1.bw" file #####

~~~{.py}
PERIODICITY 12.0
4.0 40000000
8.0 60000000
~~~

In this example, the bandwidth changes repeatedly, with all changes
being repeated every 12 seconds.

At the beginning of the the simulation, the link's bandwidth is 80,000,000
B/s (i.e., 80 Mb/s); this value was defined in the XML snippet above.
After four seconds, it drops to 40 Mb/s (line 2), and climbs
back to 60 Mb/s after another 4 seconds (line 3). The value does not change any
more until the end of the period, that is, after 12 seconds have been simulated).
At this point, periodicity kicks in and this behavior is repeated: Seconds
12-16 will experience 80 Mb/s, 16-20 40 Mb/s etc.).

##### Example of "link1.lat" file #####

~~~{.py}
PERIODICITY 5.0
1.0 0.001
2.0 0.01
3.0 0.001
~~~

In this example, the latency varies with a period of 5 seconds.
In the xml snippet above, the latency is initialized to be 0.0001s (100µs). This
value will be kept during the first second, since the latency_file contains
changes to this value at second one, two and three.
At second one, the value will be 0.001, i.e., 1ms. One second later it will
be adjusted to 0.01 (or 10ms) and one second later it will be set again to 1ms. The
value will not change until second 5, when the periodicity defined in line 1
kicks in. It then loops back, starting at 100µs (the initial value) for one second.


#### The ``<prop/>`` tag ####

Similar to ``<host>``, the link may also contain the ``<prop/>`` tag; see the host
documentation (Section \ref pf_host) for an example.


TODO

\subsubsection pf_backbone <backbone/>

\note
  This tag is <b>only available</b> when the containing AS uses the "Cluster" mode!

TODO: Is default=shared correct?

Attribute name  | Mandatory | Values | Description
--------------- | --------- | ------ | -----------
id              | yes       | string | The identifier of the link to be used when referring to it.
bandwidth       | yes       | int    | Maximum bandwidth for this link, given in bytes/s
latency         | no        | double (default: 0.0) | Latency for this link.
sharing_policy  | no        | SHARED\|FATPIPE\|FULLDUPLEX (default: SHARED) | Sharing policy for the link.

\subsection pf_storage Storage

\note
  This is a prototype version that should evolve quickly, this
  is just some doc valuable only at the time of writing this doc
  This section describes the storage management under SimGrid ; nowadays
  it's only usable with MSG. It relies basically on linux-like concepts.
  You also may want to have a look to its corresponding section in \ref
  msg_file_management ; functions access are organized as a POSIX-like
  interface.

\subsubsection pf_sto_conc Storage Main concepts
Basically there is 3 different entities to know :
\li the <b>storage_type</b>: here you define some kind of storage that
    you will instantiate many type on your platform. Think of it like
    a definition of throughput of a specific disk.
\li the <b>storage</b>: instance of a <b>storage_type</b>. Defines a
    new storage of <b>storage_type</b>
\li the <b>mount</b>: says that the storage is located into this
    specific resource.

the content of a storage has to be defined in a content file that
contains the content. The path to this file has to be passed within
the <b>content</b> attribute . Here is a way to generate it:

\verbatim
find /path/you/want -type f -exec ls -l {} \; 2>/dev/null > ./content.txt
\endverbatim

\subsubsection pf_sto_sttp storage_type


<b>storage_type</b> attributes :
\li <b>id (mandatory)</b>: the identifier of the storage_type to be
    used when referring to it.
\li <b>model (mandatory)</b>: Unused for now by the simulator (but
    mandatory, ok)
\li <b>content</b>: default value 0. The file containing the disk
    content. (may be moved soon or later to <b>storage</b> tag.

The tag must contains some predefined model prop, as may do some other
resources tags.
<b>storage_type</b> mandatory <b>model_prop</b> :
\li <b>Bwrite</b>: value in B/s. Write throughput
\li <b>Bread</b>: value in B/s. Read throughput
\li <b>Bconnexion</b>: value in B/s. Connection throughput (i.e. the
    throughput of the storage connector).

A storage_type can also contain the <b>prop</b> tag. The prop tag allows you
to define additional information on this storage_type following the
attribute/value schema. You may want to use it to give information to
the tool you use for rendering your simulation, for example.

\verbatim
<storage_type id="single_HDD" model="linear_no_lat" size="4000" content_type="txt_unix">
  <model_prop id="Bwrite" value="30MBps" />
  <model_prop id="Bread" value="100MBps" />
  <model_prop id="Bconnection" value="150MBps" />
  <b><prop id="Brand" value="Western Digital" /></b>
</storage_type>
\endverbatim

\subsubsection pf_sto_st storage

<b>storage_type</b> attributes :
\li <b>id (mandatory)</b>: the identifier of the storage to be used
    when referring to it.
\li <b>typeId (mandatory)</b>: the identifier of the storage_type that
    this storage belongs to.
\li <b>attach (mandatory)</b>: the host (name) to which the storage is
        attached to.

\subsubsection pf_sto_mo mount

<b>mount</b> attributes :
\li <b>id (mandatory)</b>: the id of the <b>storage</b> that must be
    mounted on that computer.
\li <b>name (mandatory)</b>: the name that will be the logical
    reference to this disk (the mount point).

\subsubsection pf_sto_mst mstorage
<b>Note : unused for now</b>
<b>mstorage</b> attributes :
\li <b>typeId (mandatory)</b>: the id of the <b>storage</b> that must
    be mounted on that computer.
\li <b>name (mandatory)</b>: the name that will be the logical
    reference to this disk (the mount point).

\section pf_routing Routing

To achieve high performance, the routing tables used within SimGrid are
static. This means that routing between two nodes is calculated once
and will not change during execution. The SimGrid team chose to use this
approach as it is rare to have a real deficiency of a resource;
most of the time, a communication fails because the links experience too much
congestion and hence, your connection stops before the timeout or
because the computer designated to be the destination of that message
is not responding.

We also chose to use shortest paths algorithms in order to emulate
routing. Doing so is consistent with the reality: RIP, OSPF, BGP are
all calculating shortest paths. They have some convergence time, but
at the end, so when the platform is stable (and this should be the
moment you want to simulate something using SimGrid) your packets will
follow the shortest paths.

\subsection pf_rm Routing models

Within each AS, you have to define a routing model to use. You have
basically 3 main kind of routing models :

\li Shortest-path based models: you let SimGrid calculates shortest
    paths and manage it. Behaves more or less as most real life
    routing.
\li Manually-entered route models: you'll have to define all routes
    manually by yourself into the platform description file.
    Consistent with some manually managed real life routing.
\li Simple/fast models: those models offers fast, low memory routing
    algorithms. You should consider to use it if you can make some
    assumptions about your AS. Routing in this case is more or less
    ignored

\subsubsection pf_raf The router affair

Expressing routers becomes mandatory when using shortest-path based
models or when using ns-3 or the bindings to the GTNetS packet-level
simulator instead of the native analytical network model implemented
in SimGrid.

For graph-based shortest path algorithms, routers are mandatory,
because both algorithms need a graph, and so we need to have source
and destination for each edge.

Routers are naturally an important concept in GTNetS or ns-3 since the
way they run the packet routing algorithms is actually simulated.
Instead, the SimGrid’s analytical models aggregate the routing time
with the transfer time. Rebuilding a graph representation only from
the route information turns to be a very difficult task, because of
the missing information about how routes intersect. That is why we
introduced a \<router\> tag, which is simply used to express these
intersection points. The only attribute accepted by this tag an id. It
is important to understand that the \<router\> tag is only used to
provide topological information.

To express those topological information, some <b>route</b> have to be
defined saying which link is between which routers. Description or the
route syntax is given below, as well as example for the different
models.

\subsubsection pf_rm_sh Shortest-path based models

Here is the complete list of such models, that computes routes using
classic shortest-paths algorithms. How to choose the best suited
algorithm is discussed later in the section devoted to it.

\li <b>Floyd</b>: Floyd routing data. Pre-calculates all routes once.
\li <b>Dijkstra</b>: Dijkstra routing data ,calculating routes when
    necessary.
\li <b>DijkstraCache</b>: Dijkstra routing data. Handle some cache for
    already calculated routes.

All those shortest-path models are instanciated the same way. Here are
some example of it:

Floyd example :
\verbatim
<AS  id="AS0"  routing="Floyd">

  <cluster id="my_cluster_1" prefix="c-" suffix=""
                radical="0-1"   power="1000000000"    bw="125000000"     lat="5E-5"
        router_id="router1"/>

 <AS id="AS1" routing="none">
    <host id="host1" power="1000000000"/>
 </AS>

  <link id="link1" bandwidth="100000" latency="0.01"/>

  <ASroute src="my_cluster_1" dst="AS1"
    gw_src="router1"
    gw_dst="host1">
    <link_ctn id="link1"/>
  </ASroute>

</AS>
\endverbatim

ASroute given at the end gives a topological information: link1 is
between router1 and host1.

Dijsktra example :
\verbatim
 <AS id="AS_2" routing="Dijsktra">
     <host id="AS_2_host1" power="1000000000"/>
     <host id="AS_2_host2" power="1000000000"/>
     <host id="AS_2_host3" power="1000000000"/>
     <link id="AS_2_link1" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link2" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link3" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link4" bandwidth="1250000000" latency="5E-4"/>
     <router id="central_router"/>
     <router id="AS_2_gateway"/>
     <!-- routes providing topological information -->
     <route src="central_router" dst="AS_2_host1"><link_ctn id="AS_2_link1"/></route>
     <route src="central_router" dst="AS_2_host2"><link_ctn id="AS_2_link2"/></route>
     <route src="central_router" dst="AS_2_host3"><link_ctn id="AS_2_link3"/></route>
     <route src="central_router" dst="AS_2_gateway"><link_ctn id="AS_2_link4"/></route>
  </AS>
\endverbatim

DijsktraCache example :
\verbatim
<AS id="AS_2" routing="DijsktraCache">
     <host id="AS_2_host1" power="1000000000"/>
     ...
(platform unchanged compared to upper example)
\endverbatim

\subsubsection pf_rm_me Manually-entered route models

\li <b>Full</b>: You have to enter all necessary routes manually

Full example :
\verbatim
<AS  id="AS0"  routing="Full">
   <host id="host1" power="1000000000"/>
   <host id="host2" power="1000000000"/>
   <link id="link1" bandwidth="125000000" latency="0.000100"/>
   <route src="host1" dst="host2"><link_ctn id="link1"/></route>
 </AS>
\endverbatim

\subsubsection pf_rm_sf Simple/fast models

\li <b>none</b>: No routing (Unless you know what you are doing, avoid
using this mode in combination with a non Constant network model).
None Example :
\verbatim
<AS id="exitAS"  routing="none">
        <router id="exit_gateway"/>
</AS>\endverbatim

\li <b>Vivaldi</b>: Vivaldi routing, so when you want to use
    coordinates. See the corresponding section P2P below for details.
\li <b>Cluster</b>: Cluster routing, specific to cluster tag, should
    not be used, except internally.

\subsection ps_dec Defining routes

The principle of route definition is the same for the 4 available tags
for doing it. Those for tags are:

\li <b>route</b>: to define route between host/router
\li <b>ASroute</b>: to define route between AS
\li <b>bypassRoute</b>: to bypass normal routes as calculated by the
    network model between host/router
\li <b>bypassASroute</b>: same as bypassRoute, but for AS

Basically all those tags will contain an (ordered) list of references
to link that compose the route you want to define.

Consider the example below:

\verbatim
<route src="Alice" dst="Bob">
        <link_ctn id="link1"/>
        <link_ctn id="link2"/>
        <link_ctn id="link3"/>
   </route>
\endverbatim

The route here from host Alice to Bob will be first link1, then link2,
and finally link3. What about the reverse route ? <b>route</b> and
<b>ASroute</b> have an optional attribute <b>symmetrical</b>, that can
be either YES or NO. YES means that the reverse route is the same
route in the inverse order, and is set to YES by default. Note that
this is not the case for bypass*Route, as it is more probable that you
want to bypass only one default route.

For an ASroute, things are just slightly more complicated, as you have
to give the id of the gateway which is inside the AS you're talking
about you want to access ... So it looks like this :


\verbatim
  <ASroute src="AS1" dst="AS2"
    gw_src="router1" gw_dst="router2">
    <link_ctn id="link1"/>
  </ASroute>
\endverbatim

gw == gateway, so when any message are trying to go from AS1 to AS2,
it means that it must pass through router1 to get out of the AS, then
pass through link1, and get into AS2 by being received by router2.
router1 must belong to AS1 and router2 must belong to AS2.

\subsubsection pf_linkctn link_ctn

a <b>link_ctn</b> is the tag that is used in order to reference a
<b>link</b> in a route. Its id is the link id it refers to.

<b>link_ctn</b> attributes :
\li <b>id (mandatory)</b>: Id of the link this tag refers to
\li <b>direction</b>: if the link referenced by <b>id</b> has been
    declared as FULLDUPLEX, this is used to indicate in which
    direction the route you're defining is going through this link.
    Possible values "UP" or "DOWN".

\subsubsection pf_asro ASroute

ASroute tag purpose is to let people write manually their routes
between AS. It's useful when you're in Full model.

<b>ASroute</b> attributes :
\li <b>src (mandatory)</b>: the source AS id.
\li <b>dst (mandatory)</b>: the destination AS id.
\li <b>gw_src (mandatory)</b>: the gateway to be used within the AS.
    Can be any <b>host</b> or \b router defined into the \b src AS or
    into one of the AS it includes.
\li <b>gw_dst (mandatory)</b>: the gateway to be used within the AS.
    Can be any <b>host</b> or \b router defined into the \b dst AS or
    into one of the AS it includes.
\li <b>symmetrical</b>: if the route is symmetric, the reverse route
    will be the opposite of the one defined. Can be either YES or NO,
    default is YES.

<b>Example of ASroute with Full</b>
\verbatim
<AS  id="AS0"  routing="Full">
  <cluster id="my_cluster_1" prefix="c-" suffix=".me"
                radical="0-149" power="1000000000"    bw="125000000"     lat="5E-5"
        bb_bw="2250000000" bb_lat="5E-4"/>

  <cluster id="my_cluster_2" prefix="c-" suffix=".me"
            radical="150-299" power="1000000000"        bw="125000000"  lat="5E-5"
            bb_bw="2250000000" bb_lat="5E-4"/>

     <link id="backbone" bandwidth="1250000000" latency="5E-4"/>

     <ASroute src="my_cluster_1" dst="my_cluster_2"
         gw_src="c-my_cluster_1_router.me"
         gw_dst="c-my_cluster_2_router.me">
                <link_ctn id="backbone"/>
     </ASroute>
     <ASroute src="my_cluster_2" dst="my_cluster_1"
         gw_src="c-my_cluster_2_router.me"
         gw_dst="c-my_cluster_1_router.me">
                <link_ctn id="backbone"/>
     </ASroute>
</AS>
\endverbatim

\subsubsection pf_ro route
The principle is the same as ASroute : <b>route</b> contains list of
links that are in the path between src and dst, except that it is for
routes between a src that can be either <b>host</b> or \b router and a
dst that can be either <b>host</b> or \b router. Useful for Full
as well as for the shortest-paths based models, where you
have to give topological information.


<b>route</b> attributes :
\li <b>src (mandatory)</b>: the source id.
\li <b>dst (mandatory)</b>: the destination id.
\li <b>symmetrical</b>: if the route is symmetric, the reverse route
    will be the opposite of the one defined. Can be either YES or NO,
    default is YES.

<b>route example in Full</b>
\verbatim
 <route src="Tremblay" dst="Bourassa">
     <link_ctn id="4"/><link_ctn id="3"/><link_ctn id="2"/><link_ctn id="0"/><link_ctn id="1"/><link_ctn id="6"/><link_ctn id="7"/>
   </route>
\endverbatim

<b>route example in a shortest-path model</b>
\verbatim
 <route src="Tremblay" dst="Bourassa">
     <link_ctn id="3"/>
   </route>
\endverbatim
Note that when using route to give topological information, you have
to give routes with one link only in it, as SimGrid needs to know
which host are at the end of the link.

\subsubsection pf_byASro bypassASroute

<b>Note : bypassASroute and bypassRoute are under rewriting to perform
better ; so you may not use it yet</b> As said before, once you choose
a model, it (if so) calculates routes for you. But maybe you want to
define some of your routes, which will be specific. You may also want
to bypass some routes defined in lower level AS at an upper stage :
<b>bypassASroute</b> is the tag you're looking for. It allows to
bypass routes defined between already defined between AS (if you want
to bypass route for a specific host, you should just use byPassRoute).
The principle is the same as ASroute : <b>bypassASroute</b> contains
list of links that are in the path between src and dst.

<b>bypassASroute</b> attributes :
\li <b>src (mandatory)</b>: the source AS id.
\li <b>dst (mandatory)</b>: the destination AS id.
\li <b>gw_src (mandatory)</b>: the gateway to be used within the AS.
    Can be any <b>host</b> or \b router defined into the \b src AS or
    into one of the AS it includes.
\li <b>gw_dst (mandatory)</b>: the gateway to be used within the AS.
    Can be any <b>host</b> or \b router defined into the \b dst AS or
    into one of the AS it includes.
\li <b>symmetrical</b>: if the route is symmetric, the reverse route
    will be the opposite of the one defined. Can be either YES or NO,
    default is YES.

<b>bypassASroute Example</b>
\verbatim
    <bypassASRoute src="my_cluster_1" dst="my_cluster_2"
     gw_src="my_cluster_1_router"
     gw_dst="my_cluster_2_router">
        <link_ctn id="link_tmp"/>
     </bypassASroute>
\endverbatim

\subsubsection pf_byro bypassRoute
<b>Note : bypassASRoute and bypassRoute are under rewriting to perform
better ; so you may not use it yet</b> As said before, once you choose
a model, it (if so) calculates routes for you. But maybe you want to
define some of your routes, which will be specific. You may also want
to bypass some routes defined in lower level AS at an upper stage :
<b>bypassRoute</b> is the tag you're looking for. It allows to bypass
routes defined between <b>host/router</b>. The principle is the same
as route : <b>bypassRoute</b> contains list of links references of
links that are in the path between src and dst.

<b>bypassRoute</b> attributes :
\li <b>src (mandatory)</b>: the source AS id.
\li <b>dst (mandatory)</b>: the destination AS id.
\li <b>symmetrical</b>: if the route is symmetric, the reverse route
    will be the opposite of the one defined. Can be either YES or NO,
    default is YES.

<b>bypassRoute Example</b>
\verbatim
    <bypassRoute src="host_1" dst="host_2">
        <link_ctn id="link_tmp"/>
     </bypassRoute>
\endverbatim


\subsection pb_baroex Basic Routing Example

Let's say you have an AS named AS_Big that contains two other AS, AS_1
and AS_2. If you want to make a host (h1) from AS_1 with another one
(h2) from AS_2 then you'll have to proceed as follows:
\li First, you have to ensure that a route is defined from h1 to the
    AS_1's exit gateway and from h2 to AS_2's exit gateway.
\li Then, you'll have to define a route between AS_1 to AS_2. As those
    AS are both resources belonging to AS_Big, then it has to be done
    at AS_big level. To define such a route, you have to give the
    source AS (AS_1), the destination AS (AS_2), and their respective
    gateway (as the route is effectively defined between those two
    entry/exit points). Elements of this route can only be elements
    belonging to AS_Big, so links and routers in this route should be
    defined inside AS_Big. If you choose some shortest-path model,
    this route will be computed automatically.

As said before, there are mainly 2 tags for routing :
\li <b>ASroute</b>: to define routes between two  <b>AS</b>
\li <b>route</b>: to define routes between two <b>host/router</b>

As we are dealing with routes between AS, it means that those we'll
have some definition at AS_Big level. Let consider AS_1 contains 1
host, 1 link and one router and AS_2 3 hosts, 4 links and one router.
There will be a central router, and a cross-like topology. At the end
of the crosses arms, you'll find the 3 hosts and the router that will
act as a gateway. We have to define routes inside those two AS. Let
say that AS_1 contains full routes, and AS_2 contains some Floyd
routing (as we don't want to bother with defining all routes). As
we're using some shortest path algorithms to route into AS_2, we'll
then have to define some <b>route</b> to gives some topological
information to SimGrid. Here is a file doing it all :

\verbatim
<AS  id="AS_Big"  routing="Dijsktra">
  <AS id="AS_1" routing="Full">
     <host id="AS_1_host1" power="1000000000"/>
     <link id="AS_1_link" bandwidth="1250000000" latency="5E-4"/>
     <router id="AS_1_gateway"/>
     <route src="AS_1_host1" dst="AS_1_gateway">
            <link_ctn id="AS_1_link"/>
     </route>
  </AS>
  <AS id="AS_2" routing="Floyd">
     <host id="AS_2_host1" power="1000000000"/>
     <host id="AS_2_host2" power="1000000000"/>
     <host id="AS_2_host3" power="1000000000"/>
     <link id="AS_2_link1" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link2" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link3" bandwidth="1250000000" latency="5E-4"/>
     <link id="AS_2_link4" bandwidth="1250000000" latency="5E-4"/>
     <router id="central_router"/>
     <router id="AS_2_gateway"/>
     <!-- routes providing topological information -->
     <route src="central_router" dst="AS_2_host1"><link_ctn id="AS_2_link1"/></route>
     <route src="central_router" dst="AS_2_host2"><link_ctn id="AS_2_link2"/></route>
     <route src="central_router" dst="AS_2_host3"><link_ctn id="AS_2_link3"/></route>
     <route src="central_router" dst="AS_2_gateway"><link_ctn id="AS_2_link4"/></route>
  </AS>
    <link id="backbone" bandwidth="1250000000" latency="5E-4"/>

     <ASroute src="AS_1" dst="AS_2"
         gw_src="AS_1_gateway"
         gw_dst="AS_2_gateway">
                <link_ctn id="backbone"/>
     </ASroute>
</AS>
\endverbatim

\section pf_other_tags Tags not (directly) describing the platform

There are 3 tags, that you can use inside a \<platform\> tag that are
not describing the platform:
\li random: it allows you to define random generators you want to use
    for your simulation.
\li config: it allows you to pass some configuration stuff like, for
    example, the network model and so on. It follows the
\li include: simply allows you to include another file into the
    current one.

\subsection pf_conf config
<b>config</b> attributes :
\li <b>id (mandatory)</b>: the identifier of the config to be used
    when referring to it.


<b>config</b> tag only purpose is to include <b>prop</b> tags. Valid
id are basically the same as the list of possible parameters you can
use by command line, except that "/" are used for namespace
definition. See the \ref options config and options page for more
information.


<b>config example</b>
\verbatim
<?xml version='1.0'?>
<!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
<platform version="3">
<config id="General">
        <prop id="maxmin/precision" value="0.000010"></prop>
        <prop id="cpu/optim" value="TI"></prop>
        <prop id="workstation/model" value="compound"></prop>
        <prop id="network/model" value="SMPI"></prop>
        <prop id="path" value="~/"></prop>
        <prop id="smpi/bw_factor" value="65472:0.940694;15424:0.697866;9376:0.58729"></prop>
</config>

<AS  id="AS0"  routing="Full">
...
\endverbatim


\subsection pf_rand random
Not yet in use, and possibly subject to huge modifications.

\subsection pf_incl include
<b>include</b> tag allows to import into a file platform parts located
in another file. This is done with the intention to help people
combine their different AS and provide new platforms. Those files
should contains XML part that contains either
<b>include,cluster,peer,AS,trace,trace_connect</b> tags.

<b>include</b> attributes :
\li <b>file (mandatory)</b>: filename of the file to include. Possible
    values: absolute or relative path, syntax similar to the one in
    use on your system.

<b>Note</b>: due to some obscure technical reasons, you have to open
and close tag in order to let it work.
<b>include Example</b>
\verbatim
<?xml version='1.0'?>
<!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
<platform version="3">
        <AS id="main" routing="Full">
                <include file="clusterA.xml"></include>
                <include file="clusterB.xml"></include>
        </AS>
</platform>
\endverbatim

\subsection pf_tra trace and trace_connect
Both tags are an alternate way to passe availability, state, and so on
files to entity. Instead of referring to the file directly in the host,
link, or cluster tag, you proceed by defining a trace with an id
corresponding to a file, later a host/link/cluster, and finally using
trace_connect you say that the file trace must be used by the entity.
Get it ? Let's have a look at an example :

\verbatim
<AS  id="AS0"  routing="Full">
  <host id="bob" power="1000000000"/>
</AS>
  <trace id="myTrace" file="bob.trace" periodicity="1.0"/>
  <trace_connect trace="myTrace" element="bob" kind="POWER"/>
\endverbatim

All constraints you have is that <b>trace_connect</b> is after
<b>trace</b> and <b>host</b> definitions.


<b>trace</b> attributes :
\li <b>id (mandatory)</b>: the identifier of the trace to be used when
    referring to it.
\li <b>file</b>: filename of the file to include. Possible values :
    absolute or relative path, syntax similar to the one in use on
    your system. If omitted, the system expects that you provide the
    trace values inside the trace tags (see below).
\li <b>trace periodicity (mandatory)</b>: trace periodicity, same
    definition as in hosts (see upper for details).

Here is an example  of trace when no file name is provided:

\verbatim
 <trace id="myTrace" periodicity="1.0">
    0.0 1.0
    11.0 0.5
    20.0 0.8
  </trace>
\endverbatim

<b>trace_connect</b> attributes :
\li <b>kind</b>: the type of trace, possible values
    <b>HOST_AVAIL|POWER|LINK_AVAIL|BANDWIDTH|LATENCY,</b>  default:
    <b>HOST_AVAIL</b>
\li <b>trace (mandatory)</b>: the identifier of the trace referenced.
\li <b>element (mandatory)</b>: the identifier of the entity referenced.



\section pf_hints Hints and tips, or how to write a platform efficiently

Now you should know at least the syntax and be able to create a
platform by your own. However, after having ourselves wrote some platforms, there
are some best practices you should pay attention to in order to
produce good platform and some choices you can make in order to have
faster simulations. Here's some hints and tips, then.

\subsection pf_as_h AS Hierarchy
The AS design allows SimGrid to go fast, because computing route is
done only for the set of resources defined in this AS. If you're using
only a big AS containing all resource with no AS into it and you're
using Full model, then ... you'll loose all interest into it. On the
other hand, designing a binary tree of AS with, at the lower level,
only one host, then you'll also loose all the good AS hierarchy can
give you. Remind you should always be "reasonable" in your platform
definition when choosing the hierarchy. A good choice if you try to
describe a real life platform is to follow the AS described in
reality, since this kind of trade-off works well for real life
platforms.

\subsection pf_exit_as Exit AS: why and how
Users that have looked at some of our platforms may have notice a
non-intuitive schema ... Something like that :


\verbatim
<AS id="AS_4"  routing="Full">
<AS id="exitAS_4"  routing="Full">
        <router id="router_4"/>
</AS>
<cluster id="cl_4_1" prefix="c_4_1-" suffix="" radical="1-20" power="1000000000" bw="125000000" lat="5E-5" bb_bw="2250000000" bb_lat="5E-4"/>
<cluster id="cl_4_2" prefix="c_4_2-" suffix="" radical="1-20" power="1000000000" bw="125000000" lat="5E-5" bb_bw="2250000000" bb_lat="5E-4"/>
<link id="4_1" bandwidth="2250000000" latency="5E-5"/>
<link id="4_2" bandwidth="2250000000" latency="5E-5"/>
<link id="bb_4" bandwidth="2250000000" latency="5E-4"/>
<ASroute src="cl_4_1"
        dst="cl_4_2"
        gw_src="c_4_1-cl_4_1_router"
        gw_dst="c_4_2-cl_4_2_router"
        symmetrical="YES">
                <link_ctn id="4_1"/>
                <link_ctn id="bb_4"/>
                <link_ctn id="4_2"/>
</ASroute>
<ASroute src="cl_4_1"
        dst="exitAS_4"
        gw_src="c_4_1-cl_4_1_router"
        gw_dst="router_4"
        symmetrical="YES">
                <link_ctn id="4_1"/>
                <link_ctn id="bb_4"/>
</ASroute>
<ASroute src="cl_4_2"
        dst="exitAS_4"
        gw_src="c_4_2-cl_4_2_router"
        gw_dst="router_4"
        symmetrical="YES">
                <link_ctn id="4_2"/>
                <link_ctn id="bb_4"/>
</ASroute>
</AS>
\endverbatim

In the AS_4, you have an exitAS_4 defined, containing only one router,
and routes defined to that AS from all other AS (as cluster is only a
shortcut for an AS, see cluster description for details). If there was
an upper AS, it would define routes to and from AS_4 with the gateway
router_4. It's just because, as we did not allowed (for performances
issues) to have routes from an AS to a single host/router, you have to
enclose your gateway, when you have AS included in your AS, within an
AS to define routes to it.

\subsection pf_P2P_tags P2P or how to use coordinates
SimGrid allows you to use some coordinated-based system, like vivaldi,
to describe a platform. The main concept is that you have some peers
that are located somewhere: this is the function of the
<b>coordinates</b> of the \<peer\> or \<host\> tag. There's nothing
complicated in using it, here is an example of it:

\verbatim
<?xml version='1.0'?>
<!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
<platform version="3">

<config id="General">
        <prop id="network/coordinates" value="yes"></prop>
</config>
 <AS  id="AS0"  routing="Vivaldi">
        <host id="100030591" coordinates="25.5 9.4 1.4" power="1500000000.0" />
        <host id="100036570" coordinates="-12.7 -9.9 2.1" power="730000000.0" />
        ...
        <host id="100429957" coordinates="17.5 6.7 18.8" power="830000000.0" />
        </AS>
</platform>
\endverbatim

Coordinates are then used to calculate latency between two hosts by
calculating the euclidean distance between the two hosts coordinates.
The results express the latency in ms.

Note that the previous example defines a routing directly between hosts but it could be also used to define a routing between AS.
That is for example what is commonly done when using peers (see Section \ref pf_peer).
\verbatim
<?xml version='1.0'?>
<!DOCTYPE platform SYSTEM "http://simgrid.gforge.inria.fr/simgrid.dtd">
<platform version="3">

<config id="General">
        <prop id="network/coordinates" value="yes"></prop>
</config>
 <AS  id="AS0"  routing="Vivaldi">
   <peer id="peer-0" coordinates="173.0 96.8 0.1" power="730Mf" bw_in="13.38MBps" bw_out="1.024MBps" lat="500us"/>
   <peer id="peer-1" coordinates="247.0 57.3 0.6" power="730Mf" bw_in="13.38MBps" bw_out="1.024MBps" lat="500us" />
   <peer id="peer-2" coordinates="243.4 58.8 1.4" power="730Mf" bw_in="13.38MBps" bw_out="1.024MBps" lat="500us" />
</AS>
</platform>
\endverbatim
In such a case though, we connect the AS created by the <b>peer</b> tag with the Vivaldi routing mechanism.
This means that to route between AS1 and AS2, it will use the coordinates of router_AS1 and router_AS2.
This is currently a convention and we may offer to change this convention in the DTD later if needed.
You may have noted that conveniently, a peer named FOO defines an AS named FOO and a router named router_FOO, which is why it works seamlessly with the <b>peer</b> tag.


\subsection pf_wisely Choosing wisely the routing model to use


Choosing wisely the routing model to use can significantly fasten your
simulation/save your time when writing the platform/save tremendous
disk space. Here is the list of available model and their
characteristics (lookup : time to resolve a route):

\li <b>Full</b>: Full routing data (fast, large memory requirements,
    fully expressive)
\li <b>Floyd</b>: Floyd routing data (slow initialization, fast
    lookup, lesser memory requirements, shortest path routing only).
    Calculates all routes at once at the beginning.
\li <b>Dijkstra</b>: Dijkstra routing data (fast initialization, slow
    lookup, small memory requirements, shortest path routing only).
    Calculates a route when necessary.
\li <b>DijkstraCache</b>: Dijkstra routing data (fast initialization,
    fast lookup, small memory requirements, shortest path routing
    only). Same as Dijkstra, except it handles a cache for latest used
    routes.
\li <b>none</b>: No routing (usable with Constant network only).
    Defines that there is no routes, so if you try to determine a
    route without constant network within this AS, SimGrid will raise
    an exception.
\li <b>Vivaldi</b>: Vivaldi routing, so when you want to use coordinates
\li <b>Cluster</b>: Cluster routing, specific to cluster tag, should
    not be used.

\subsection pf_switch Hey, I want to describe a switch but there is no switch tag !

Actually we did not include switch tag, ok. But when you're trying to
simulate a switch, the only major impact it has when you're using
fluid model (and SimGrid uses fluid model unless you activate GTNetS,
ns-3, or constant network mode) is the impact of the upper limit of
the switch motherboard speed that will eventually be reached if you're
using intensively your switch. So, the switch impact is similar to a
link one. That's why we are used to describe a switch using a link tag
(as a link is not an edge by a hyperedge, you can connect more than 2
other links to it).

\subsection pf_platform_multipath How to express multipath routing in platform files?

It is unfortunately impossible to express the fact that there is more
than one routing path between two given hosts. Let's consider the
following platform file:

\verbatim
<route src="A" dst="B">
   <link_ctn id="1"/>
</route>
<route src="B" dst="C">
  <link_ctn id="2"/>
</route>
<route src="A" dst="C">
  <link_ctn id="3"/>
</route>
\endverbatim

Although it is perfectly valid, it does not mean that data traveling
from A to C can either go directly (using link 3) or through B (using
links 1 and 2). It simply means that the routing on the graph is not
trivial, and that data do not following the shortest path in number of
hops on this graph. Another way to say it is that there is no implicit
in these routing descriptions. The system will only use the routes you
declare (such as &lt;route src="A" dst="C"&gt;&lt;link_ctn
id="3"/&gt;&lt;/route&gt;), without trying to build new routes by aggregating
the provided ones.

You are also free to declare platform where the routing is not
symmetric. For example, add the following to the previous file:

\verbatim
<route src="C" dst="A">
  <link_ctn id="2"/>
  <link_ctn id="1"/>
</route>
\endverbatim

This makes sure that data from C to A go through B where data from A
to C go directly. Don't worry about realism of such settings since
we've seen ways more weird situation in real settings (in fact, that's
the realism of very regular platforms which is questionable, but
that's another story).

\section pf_flexml_bypassing Bypassing the XML parser with your own C functions
<b>NOTE THAT THIS DOCUMENTATION, WHILE STILL WORKING, IS STRONGLY DEPRECATED</b>

So you want to bypass the XML files parser, uh? Maybe doing some parameter
sweep experiments on your simulations or so? This is possible, and
it's not even really difficult (well. Such a brutal idea could be
harder to implement). Here is how it goes.

For this, you have to first remember that the XML parsing in SimGrid is done
using a tool called FleXML. Given a DTD, this gives a flex-based parser. If
you want to bypass the parser, you need to provide some code mimicking what
it does and replacing it in its interactions with the SURF code. So, let's
have a look at these interactions.

FleXML parser are close to classical SAX parsers. It means that a
well-formed SimGrid platform XML file might result in the following
"events":

  - start "platform_description" with attribute version="2"
  - start "host" with attributes id="host1" power="1.0"
  - end "host"
  - start "host" with attributes id="host2" power="2.0"
  - end "host"
  - start "link" with ...
  - end "link"
  - start "route" with ...
  - start "link_ctn" with ...
  - end "link_ctn"
  - end "route"
  - end "platform_description"

The communication from the parser to the SURF code uses two means:
Attributes get copied into some global variables, and a surf-provided
function gets called by the parser for each event. For example, the event
  - start "host" with attributes id="host1" power="1.0"

let the parser do something roughly equivalent to:
\verbatim
  strcpy(A_host_id,"host1");
  A_host_power = 1.0;
  STag_host();
\endverbatim

In SURF, we attach callbacks to the different events by initializing the
pointer functions to some the right surf functions. Since there can be
more than one callback attached to the same event (if more than one
model is in use, for example), they are stored in a dynar. Example in
workstation_ptask_L07.c:
\verbatim
  /* Adding callback functions */
  surf_parse_reset_parser();
  surfxml_add_callback(STag_surfxml_host_cb_list, &parse_cpu_init);
  surfxml_add_callback(STag_surfxml_prop_cb_list, &parse_properties);
  surfxml_add_callback(STag_surfxml_link_cb_list, &parse_link_init);
  surfxml_add_callback(STag_surfxml_route_cb_list, &parse_route_set_endpoints);
  surfxml_add_callback(ETag_surfxml_link_c_ctn_cb_list, &parse_route_elem);
  surfxml_add_callback(ETag_surfxml_route_cb_list, &parse_route_set_route);

  /* Parse the file */
  surf_parse_open(file);
  xbt_assert(!surf_parse(), "Parse error in %s", file);
  surf_parse_close();
\endverbatim

So, to bypass the FleXML parser, you need to write your own version of the
surf_parse function, which should do the following:
   - Fill the A_<tag>_<attribute> variables with the wanted values
   - Call the corresponding STag_<tag>_fun function to simulate tag start
   - Call the corresponding ETag_<tag>_fun function to simulate tag end
   - (do the same for the next set of values, and loop)

Then, tell SimGrid that you want to use your own "parser" instead of the stock one:
\verbatim
  surf_parse = surf_parse_bypass_environment;
  MSG_create_environment(NULL);
  surf_parse = surf_parse_bypass_application;
  MSG_launch_application(NULL);
\endverbatim

A set of macros are provided at the end of
include/surf/surfxml_parse.h to ease the writing of the bypass
functions. An example of this trick is distributed in the file
examples/msg/masterslave/masterslave_bypass.c


*/