add link + fix doc

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/*! \page options Simgrid options and configurations

A number of options can be given at runtime to change the default
SimGrid behavior. For a complete list of all configuration options
accepted by the SimGrid version used in your simulator, simply pass
the --help configuration flag to your program. If some of the options
are not documented on this page, this is a bug that you should please
report so that we can fix it. Note that some of the options presented
here may not be available in your simulators, depending on the 
@ref install_src_config "compile-time options" that you used.

\section options_using Passing configuration options to the simulators

There is several way to pass configuration options to the simulators.
The most common way is to use the \c --cfg command line argument. For
example, to set the item \c Item to the value \c Value, simply
type the following: \verbatim
my_simulator --cfg=Item:Value (other arguments)
\endverbatim

Several \c --cfg command line arguments can naturally be used. If you
need to include spaces in the argument, don't forget to quote the
argument. You can even escape the included quotes (write \' for ' if
you have your argument between ').

Another solution is to use the \c \<config\> tag in the platform file. The
only restriction is that this tag must occure before the first
platform element (be it \c \<AS\>, \c \<cluster\>, \c \<peer\> or whatever).
The \c \<config\> tag takes an \c id attribute, but it is currently
ignored so you don't really need to pass it. The important par is that
within that tag, you can pass one or several \c \<prop\> tags to specify
the configuration to use. For example, setting \c Item to \c Value
can be done by adding the following to the beginning of your platform
file: \verbatim
<config>
  <prop id="Item" value="Value"/>
</config>
\endverbatim

A last solution is to pass your configuration directly using the C
interface. If you happen to use the MSG interface, this is very easy
with the MSG_config() function. If you do not use MSG, that's a bit
more complex, as you have to mess with the internal configuration set
directly as follows. Check the \ref XBT_config "relevant page" for
details on all the functions you can use in this context, \c
_sg_cfg_set being the only configuration set currently used in
SimGrid. 

@code
#include <xbt/config.h>

extern xbt_cfg_t _sg_cfg_set;

int main(int argc, char *argv[]) {
     SD_init(&argc, argv);

     /* Prefer MSG_config() if you use MSG!! */
     xbt_cfg_set_parse(_sg_cfg_set,"Item:Value");

     // Rest of your code
}
@endcode

\section options_model Configuring the platform models

\subsection options_model_select Selecting the platform models

SimGrid comes with several network and CPU models built in, and you
can change the used model at runtime by changing the passed
configuration. The three main configuration items are given below.
For each of these items, passing the special \c help value gives
you a short description of all possible values. Also, \c --help-models
should provide information about all models for all existing resources.
   - \b network/model: specify the used network model
   - \b cpu/model: specify the used CPU model
   - \b workstation/model: specify the used workstation model

As of writting, the accepted network models are the following. Over
the time new models can be added, and some experimental models can be
removed; check the values on your simulators for an uptodate
information. Note that the CM02 model is described in the research report
<a href="ftp://ftp.ens-lyon.fr/pub/LIP/Rapports/RR/RR2002/RR2002-40.ps.gz">A
Network Model for Simulation of Grid Application</a> while LV08 is
described in
<a href="http://mescal.imag.fr/membres/arnaud.legrand/articles/simutools09.pdf">Accuracy Study and Improvement of Network Simulation in the SimGrid Framework</a>.

  - \b LV08 (default one): Realistic network analytic model
    (slow-start modeled by multiplying latency by 10.4, bandwidth by
    .92; bottleneck sharing uses a payload of S=8775 for evaluating RTT)
  - \b Constant: Simplistic network model where all communication
    take a constant time (one second). This model provides the lowest
    realism, but is (marginally) faster.
  - \b SMPI: Realistic network model specifically tailored for HPC
    settings (accurate modeling of slow start with correction factors on
    three intervals: < 1KiB, < 64 KiB, >= 64 KiB). See also \ref
    options_model_network_coefs "this section" for more info.
  - \b CM02: Legacy network analytic model (Very similar to LV08, but
    without corrective factors. The timings of small messages are thus
    poorly modeled)
  - \b Reno: Model from Steven H. Low using lagrange_solve instead of
    lmm_solve (experts only; check the code for more info).
  - \b Reno2: Model from Steven H. Low using lagrange_solve instead of
    lmm_solve (experts only; check the code for more info).
  - \b Vegas: Model from Steven H. Low using lagrange_solve instead of
    lmm_solve (experts only; check the code for more info).

If you compiled SimGrid accordingly, you can use packet-level network
simulators as network models (see \ref pls). In that case, you have
two extra models, described below, and some \ref options_pls "specific
additional configuration flags".
  - \b GTNets: Network pseudo-model using the GTNets simulator instead
    of an analytic model
  - \b NS3: Network pseudo-model using the NS3 tcp model instead of an
    analytic model

Concerning the CPU, we have only one model for now:
  - \b Cas01: Simplistic CPU model (time=size/power)

The workstation concept is the aggregation of a CPU with a network
card. Three models exists, but actually, only 2 of them are
interesting. The "compound" one is simply due to the way our internal
code is organized, and can easily be ignored. So at the end, you have
two workstation models: The default one allows to aggregate an
existing CPU model with an existing network model, but does not allow
parallel tasks because these beasts need some collaboration between
the network and CPU model. That is why, ptask_07 is used by default
when using SimDag.
  - \b default: Default workstation model. Currently, CPU:Cas01 and
    network:LV08 (with cross traffic enabled)
  - \b compound: Workstation model that is automatically chosen if
    you change the network and CPU models
  - \b ptask_L07: Workstation model somehow similar to Cas01+CM02 but
    allowing parallel tasks

\subsection options_model_optim Optimization level of the platform models

The network and CPU models that are based on lmm_solve (that
is, all our analytical models) accept specific optimization
configurations.
  - items \b network/optim and \b CPU/optim (both default to 'Lazy'):
    - \b Lazy: Lazy action management (partial invalidation in lmm +
      heap in action remaining).
    - \b TI: Trace integration. Highly optimized mode when using
      availability traces (only available for the Cas01 CPU model for
      now).
    - \b Full: Full update of remaining and variables. Slow but may be
      useful when debugging.
  - items \b network/maxmin_selective_update and
    \b cpu/maxmin_selective_update: configure whether the underlying
    should be lazily updated or not. It should have no impact on the
    computed timings, but should speed up the computation.

It is still possible to disable the \c maxmin_selective_update feature
because it can reveal counter-productive in very specific scenarios
where the interaction level is high. In particular, if all your
communication share a given backbone link, you should disable it:
without \c maxmin_selective_update, every communications are updated
at each step through a simple loop over them. With that feature
enabled, every communications will still get updated in this case
(because of the dependency induced by the backbone), but through a
complicated pattern aiming at following the actual dependencies.

\subsection options_model_precision Numerical precision of the platform models

The analytical models handle a lot of floating point values. It is
possible to change the epsilon used to update and compare them through
the \b maxmin/precision item (default value: 0.00001). Changing it
may speedup the simulation by discarding very small actions, at the
price of a reduced numerical precision.

\subsection options_model_nthreads Parallel threads for model updates

By default, Surf computes the analytical models sequentially to share their
resources and update their actions. It is possible to run them in parallel,
using the \b surf/nthreads item (default value: 1). If you use a
negative or null value, the amount of available cores is automatically
detected  and used instead.

Depending on the workload of the models and their complexity, you may get a
speedup or a slowdown because of the synchronization costs of threads.

\subsection options_model_network Configuring the Network model

\subsubsection options_model_network_gamma Maximal TCP window size

The analytical models need to know the maximal TCP window size to take
the TCP congestion mechanism into account. This is set to 20000 by
default, but can be changed using the \b network/TCP_gamma item.

On linux, this value can be retrieved using the following
commands. Both give a set of values, and you should use the last one,
which is the maximal size.\verbatim
cat /proc/sys/net/ipv4/tcp_rmem # gives the sender window
cat /proc/sys/net/ipv4/tcp_wmem # gives the receiver window
\endverbatim

\subsubsection options_model_network_coefs Corrective simulation factors

These factors allow to betterly take the slow start into account.
The corresponding values were computed through data fitting one the
timings of packet-level simulators. You should not change these values
unless you are really certain of what you are doing. See
<a href="http://mescal.imag.fr/membres/arnaud.legrand/articles/simutools09.pdf">Accuracy Study and Improvement of Network Simulation in the SimGrid Framework</a>
for more informations about these coeficients.

If you are using the SMPI model, these correction coeficients are
themselves corrected by constant values depending on the size of the
exchange. Again, only hardcore experts should bother about this fact.

\subsubsection options_model_network_crosstraffic Simulating cross-traffic

As of SimGrid v3.7, cross-traffic effects can be taken into account in
analytical simulations. It means that ongoing and incoming
communication flows are treated independently. In addition, the LV08
model adds 0.05 of usage on the opposite direction for each new
created flow. This can be useful to simulate some important TCP
phenomena such as ack compression.

For that to work, your platform must have two links for each
pair of interconnected hosts. An example of usable platform is
available in <tt>examples/msg/gtnets/crosstraffic-p.xml</tt>.

This is activated through the \b network/crosstraffic item, that
can be set to 0 (disable this feature) or 1 (enable it).

Note that with the default workstation model this option is activated by default.

\subsubsection options_model_network_coord Coordinated-based network models

When you want to use network coordinates, as it happens when you use
an \<AS\> in your platform file with \c Vivaldi as a routing, you must
set the \b network/coordinates to \c yes so that all mandatory
initialization are done in the simulator.

\subsubsection options_model_network_sendergap Simulating sender gap

(this configuration item is experimental and may change or disapear)

It is possible to specify a timing gap between consecutive emission on
the same network card through the \b network/sender_gap item. This
is still under investigation as of writting, and the default value is
to wait 10 microseconds (1e-5 seconds) between emissions.

\subsubsection options_model_network_asyncsend Simulating asyncronous send

(this configuration item is experimental and may change or disapear)

It is possible to specify that messages below a certain size will be sent 
as soon as the call to MPI_Send is issued, without waiting for the 
correspondant receive. This threshold can be configured through the 
\b smpi/async_small_thres item. The default value is 0. This behavior can also be 
manually set for MSG mailboxes, by setting the receiving mode of the mailbox 
with a call to \ref MSG_mailbox_set_async . For MSG, all messages sent to this 
mailbox will have this behavior, so consider using two mailboxes if needed. 

This value needs to be smaller than or equals to the threshold set at 
\ref options_model_smpi_detached , because asynchronous messages are 
meant to be detached as well.

\subsubsection options_pls Configuring packet-level pseudo-models

When using the packet-level pseudo-models, several specific
configuration flags are provided to configure the associated tools.
There is by far not enough such SimGrid flags to cover every aspects
of the associated tools, since we only added the items that we
needed ourselves. Feel free to request more items (or even better:
provide patches adding more items).

When using NS3, the only existing item is \b ns3/TcpModel,
corresponding to the ns3::TcpL4Protocol::SocketType configuration item
in NS3. The only valid values (enforced on the SimGrid side) are
'NewReno' or 'Reno' or 'Tahoe'.

When using GTNeTS, two items exist:
 - \b gtnets/jitter, that is a double value to oscillate
   the link latency, uniformly in random interval
   [-latency*gtnets_jitter,latency*gtnets_jitter). It defaults to 0.
 - \b gtnets/jitter_seed, the positive seed used to reproduce jitted
   results. Its value must be in [1,1e8] and defaults to 10.

\section options_modelchecking Configuring the Model-Checking

To enable the experimental SimGrid model-checking support the program should
be executed with the command line argument
\verbatim
--cfg=model-check:1
\endverbatim
Safety properties are expressed as assertions using the function
\verbatim
void MC_assert(int prop);
\endverbatim

\subsection options_modelchecking_liveness Specifying a liveness property

If you want to specify liveness properties (beware, that's
experimental), you have to pass them on the command line, specifying
the name of the file containing the property, as formated by the
ltl2ba program.

\verbatim
--cfg=model-check/property:<filename>
\endverbatim

Of course, specifying a liveness property enables the model-checking
so that you don't have to give <tt>--cfg=model-check:1</tt> in
addition.

\subsection options_modelchecking_steps Going for stateful verification

By default, the system is backtracked to its initial state to explore
another path instead of backtracking to the exact step before the fork
that we want to explore (this is called stateless verification). This
is done this way because saving intermediate states can rapidly
exhaust the available memory. If you want, you can change the value of
the <tt>model-check/checkpoint</tt> variable. For example, the
following configuration will ask to take a checkpoint every step.
Beware, this will certainly explode your memory. Larger values are
probably better, make sure to experiment a bit to find the right
setting for your specific system.

\verbatim
--cfg=model-check/checkpoint:1
\endverbatim

Of course, specifying this option enables the model-checking so that
you don't have to give <tt>--cfg=model-check:1</tt> in addition.

\subsection options_modelchecking_reduction Specifying the kind of reduction

The main issue when using the model-checking is the state space
explosion. To counter that problem, several exploration reduction
techniques can be used. There is unfortunately no silver bullet here,
and the most efficient reduction techniques cannot be applied to any
properties. In particular, the DPOR method cannot be applied on
liveness properties since it may break some cycles in the exploration
that are important to the property validity.

\verbatim
--cfg=model-check/reduction:<technique>
\endverbatim

For now, this configuration variable can take 2 values:
 * none: Do not apply any kind of reduction (mandatory for now for
   liveness properties)
 * dpor: Apply Dynamic Partial Ordering Reduction. Only valid if you
   verify local safety properties.

Of course, specifying a reduction technique enables the model-checking
so that you don't have to give <tt>--cfg=model-check:1</tt> in
addition.

\section options_virt Configuring the User Process Virtualization

\subsection options_virt_factory Selecting the virtualization factory

In SimGrid, the user code is virtualized in a specific mecanism
allowing the simulation kernel to control its execution: when a user
process requires a blocking action (such as sending a message), it is
interrupted, and only gets released when the simulated clock reaches
the point where the blocking operation is done.

In SimGrid, the containers in which user processes are virtualized are
called contexts. Several context factory are provided, and you can
select the one you want to use with the \b contexts/factory
configuration item. Some of the following may not exist on your
machine because of portability issues. In any case, the default one
should be the most effcient one (please report bugs if the
auto-detection fails for you). They are sorted here from the slowest
to the most effient:
 - \b thread: very slow factory using full featured threads (either
   pthreads or windows native threads)
 - \b ucontext: fast factory using System V contexts (or a portability
   layer of our own on top of Windows fibers)
 - \b raw: amazingly fast factory using a context switching mecanism
   of our own, directly implemented in assembly (only available for x86
   and amd64 platforms for now)

The only reason to change this setting is when the debugging tools get
fooled by the optimized context factories. Threads are the most
debugging-friendly contextes.

\subsection options_virt_stacksize Adapting the used stack size

Each virtualized used process is executed using a specific system
stack. The size of this stack has a huge impact on the simulation
scalability, but its default value is rather large. This is because
the error messages that you get when the stack size is too small are
rather disturbing: this leads to stack overflow (overwriting other
stacks), leading to segfaults with corrupted stack traces.

If you want to push the scalability limits of your code, you really
want to reduce the \b contexts/stack_size item. Its default value
is 128 (in KiB), while our Chord simulation works with stacks as small
as 16 KiB, for example. For the thread factory, the default value 
is the one of the system, if it is too large/small, it has to be set 
with this parameter.

\subsection options_virt_parallel Running user code in parallel

Parallel execution of the user code is only considered stable in
SimGrid v3.7 and higher. It is described in
<a href="http://hal.inria.fr/inria-00602216/">INRIA RR-7653</a>.

If you are using the \c ucontext or \c raw context factories, you can
request to execute the user code in parallel. Several threads are
launched, each of them handling as much user contexts at each run. To
actiave this, set the \b contexts/nthreads item to the amount of
cores that you have in your computer (or lower than 1 to have 
the amount of cores auto-detected).

Even if you asked several worker threads using the previous option,
you can request to start the parallel execution (and pay the
associated synchronization costs) only if the potential parallelism is
large enough. For that, set the \b contexts/parallel_threshold
item to the minimal amount of user contexts needed to start the
parallel execution. In any given simulation round, if that amount is
not reached, the contexts will be run sequentially directly by the
main thread (thus saving the synchronization costs). Note that this
option is mainly useful when the grain of the user code is very fine,
because our synchronization is now very efficient.

When parallel execution is activated, you can choose the
synchronization schema used with the \b contexts/synchro item,
which value is either:
 - \b futex: ultra optimized synchronisation schema, based on futexes
   (fast user-mode mutexes), and thus only available on Linux systems.
   This is the default mode when available.
 - \b posix: slow but portable synchronisation using only POSIX
   primitives.
 - \b busy_wait: not really a synchronisation: the worker threads
   constantly request new contexts to execute. It should be the most
   efficient synchronisation schema, but it loads all the cores of your
   machine for no good reason. You probably prefer the other less
   eager schemas.

\section options_tracing Configuring the tracing subsystem

The \ref tracing "tracing subsystem" can be configured in several
different ways depending on the nature of the simulator (MSG, SimDag,
SMPI) and the kind of traces that need to be obtained. See the \ref
tracing_tracing_options "Tracing Configuration Options subsection" to
get a detailed description of each configuration option.

We detail here a simple way to get the traces working for you, even if
you never used the tracing API.


- Any SimGrid-based simulator (MSG, SimDag, SMPI, ...) and raw traces:
\verbatim
--cfg=tracing:yes --cfg=tracing/uncategorized:yes --cfg=triva/uncategorized:uncat.plist
\endverbatim
    The first parameter activates the tracing subsystem, the second
    tells it to trace host and link utilization (without any
    categorization) and the third creates a graph configuration file
    to configure Triva when analysing the resulting trace file.

- MSG or SimDag-based simulator and categorized traces (you need to declare categories and classify your tasks according to them)
\verbatim
--cfg=tracing:yes --cfg=tracing/categorized:yes --cfg=triva/categorized:cat.plist
\endverbatim
    The first parameter activates the tracing subsystem, the second
    tells it to trace host and link categorized utilization and the
    third creates a graph configuration file to configure Triva when
    analysing the resulting trace file.

- SMPI simulator and traces for a space/time view:
\verbatim
smpirun -trace ...
\endverbatim
    The <i>-trace</i> parameter for the smpirun script runs the
simulation with --cfg=tracing:yes and --cfg=tracing/smpi:yes. Check the
smpirun's <i>-help</i> parameter for additional tracing options.

Sometimes you might want to put additional information on the trace to
correctly identify them later, or to provide data that can be used to
reproduce an experiment. You have two ways to do that:

- Add a string on top of the trace file as comment:
\verbatim
--cfg=tracing/comment:my_simulation_identifier
\endverbatim

- Add the contents of a textual file on top of the trace file as comment:
\verbatim
--cfg=tracing/comment_file:my_file_with_additional_information.txt
\endverbatim

Please, use these two parameters (for comments) to make reproducible
simulations. For additional details about this and all tracing
options, check See the \ref tracing_tracing_options.

\section options_smpi Configuring SMPI

The SMPI interface provides several specific configuration items.
These are uneasy to see since the code is usually launched through the
\c smiprun script directly.

\subsection options_smpi_bench Automatic benchmarking of SMPI code

In SMPI, the sequential code is automatically benchmarked, and these
computations are automatically reported to the simulator. That is to
say that if you have a large computation between a \c MPI_Recv() and a
\c MPI_Send(), SMPI will automatically benchmark the duration of this
code, and create an execution task within the simulator to take this
into account. For that, the actual duration is measured on the host
machine and then scaled to the power of the corresponding simulated
machine. The variable \b smpi/running_power allows to specify the
computational power of the host machine (in flop/s) to use when
scaling the execution times. It defaults to 20000, but you really want
to update it to get accurate simulation results.

When the code is constituted of numerous consecutive MPI calls, the
previous mechanism feeds the simulation kernel with numerous tiny
computations. The \b smpi/cpu_threshold item becomes handy when this
impacts badly the simulation performance. It specify a threshold (in
second) under which the execution chunks are not reported to the
simulation kernel (default value: 1e-6). Please note that in some
circonstances, this optimization can hinder the simulation accuracy.

\subsection options_smpi_timing Reporting simulation time

Most of the time, you run MPI code through SMPI to compute the time it
would take to run it on a platform that you don't have. But since the
code is run through the \c smpirun script, you don't have any control
on the launcher code, making difficult to report the simulated time
when the simulation ends. If you set the \b smpi/display_timing item
to 1, \c smpirun will display this information when the simulation ends. \verbatim
Simulation time: 1e3 seconds.
\endverbatim

\subsection options_model_smpi_detached Simulating MPI detached send

(this configuration item is experimental and may change or disapear)

This threshold specifies the size in bytes under which the send will return 
immediately. This is different from the threshold detailed in  \ref options_model_network_asyncsend
because the message is not effectively sent when the send is posted. SMPI still waits for the
correspondant receive to be posted to perform the communication operation. This threshold can be set 
by changing the \b smpi/send_is_detached item. The default value is 65536.

\subsection options_model_smpi_collectives Simulating MPI collective algorithms

SMPI implements more than 100 different algorithms for MPI collective communication, to accurately 
simulate the behavior of most of the existing MPI libraries. The \b smpi/coll_selector item can be used
 to use the decision logic of either OpenMPI or MPICH libraries (values: ompi or mpich, by default SMPI
uses naive version of collective operations). Each collective operation can be manually selected with a 
\b smpi/collective_name:algo_name. Available algorithms are listed in \ref SMPI_collective_algorithms .

\section options_generic Configuring other aspects of SimGrid

\subsection options_generic_path XML file inclusion path

It is possible to specify a list of directories to search into for the
\<include\> tag in XML files by using the \b path configuration
item. To add several directory to the path, set the configuration
item several times, as in \verbatim
--cfg=path:toto --cfg=path:tutu
\endverbatim

\subsection options_generic_exit Behavior on Ctrl-C

By default, when Ctrl-C is pressed, the status of all existing
simulated processes is displayed. This is very useful to debug your
code, but it can reveal troublesome in some cases (such as when the
amount of processes becomes really big). This behavior is disabled
when \b verbose-exit is set to 0 (it is to 1 by default).


\section options_log Logging Configuration

It can be done by using XBT. Go to \ref XBT_log for more details.


\section options_index Index of all existing configuration items

- \c contexts/factory: \ref options_virt_factory
- \c contexts/nthreads: \ref options_virt_parallel
- \c contexts/parallel_threshold: \ref options_virt_parallel
- \c contexts/stack_size: \ref options_virt_stacksize
- \c contexts/synchro: \ref options_virt_parallel

- \c cpu/maxmin_selective_update: \ref options_model_optim
- \c cpu/model: \ref options_model_select
- \c cpu/optim: \ref options_model_optim

- \c gtnets/jitter: \ref options_pls
- \c gtnets/jitter_seed: \ref options_pls

- \c maxmin/precision: \ref options_model_precision

- \c model-check: \ref options_modelchecking
- \c model-check/property: \ref options_modelchecking_liveness
- \c model-check/checkpoint: \ref options_modelchecking_steps
- \c model-check/reduce: \ref options_modelchecking_reduction

- \c network/bandwidth_factor: \ref options_model_network_coefs
- \c network/coordinates: \ref options_model_network_coord
- \c network/crosstraffic: \ref options_model_network_crosstraffic
- \c network/latency_factor: \ref options_model_network_coefs
- \c network/maxmin_selective_update: \ref options_model_optim
- \c network/model: \ref options_model_select
- \c network/optim: \ref options_model_optim
- \c network/sender_gap: \ref options_model_network_sendergap
- \c network/TCP_gamma: \ref options_model_network_gamma
- \c network/weight_S: \ref options_model_network_coefs

- \c ns3/TcpModel: \ref options_pls

- \c surf/nthreads: \ref options_model_nthreads

- \c smpi/running_power: \ref options_smpi_bench
- \c smpi/display_timing: \ref options_smpi_timing
- \c smpi/cpu_threshold: \ref options_smpi_bench
- \c smpi/async_small_thres: \ref options_model_network_asyncsend
- \c smpi/send_is_detached: \ref options_model_smpi_detached
- \c smpi/coll_selector: \ref options_model_smpi_collectives

- \c path: \ref options_generic_path
- \c verbose-exit: \ref options_generic_exit

- \c workstation/model: \ref options_model_select

*/