[doc] explain how to use TRACE_declare_mark and TRACE_mark

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/*! \page options Simgrid options and configurations

\htmlinclude .options.doc.toc

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.

\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. Unfortunately, this path is not really easy to use right
now, and you mess directly with surf internal variables as follows. Check the
\ref XBT_config "relevant page" for details on all the functions you
can use in this context, \c _surf_cfg_set being the only configuration set
currently used in SimGrid. \code
#include <xbt/config.h>

extern xbt_cfg_t _surf_cfg_set;

int main(int argc, char *argv[]) {
     MSG_global_init(&argc, argv);
     
     xbt_cfg_set_parse(_surf_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: 1e-9). Changing it
may speedup the simulation by discarding very small actions, at the
price of a reduced numerical precision.

\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).

\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 0 seconds between emissions (no gap applied).

\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
Properties are expressed as assertions using the function
\verbatim
void MC_assert(int prop);
\endverbatim

\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
   ptheads 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 debuging tools get
fooled by the optimized context factories. Threads are the most
debugging-friendly contextes.

\subsection options_virt_stacksize Adapting the used stack size

(this only works if you use ucontexts or raw context factories)

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.

\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
core that you have in your computer.

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

(TODO)

\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

\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_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 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 smpi/running_power: \ref options_smpi_bench
- \c smpi/display_timing: \ref options_smpi_timing
- \c smpi/cpu_threshold: \ref options_smpi_bench

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

- \c workstation/model: \ref options_model_select

*/