starpu-max/doc/doxygen/chapters/15environment_variables.doxy

/*
 * This file is part of the StarPU Handbook.
 * Copyright (C) 2009--2011  Universit@'e de Bordeaux 1
 * Copyright (C) 2010, 2011, 2012, 2013  Centre National de la Recherche Scientifique
 * Copyright (C) 2011, 2012 Institut National de Recherche en Informatique et Automatique
 * See the file version.doxy for copying conditions.
 */

/*! \page ExecutionConfigurationThroughEnvironmentVariables Execution Configuration Through Environment Variables

The behavior of the StarPU library and tools may be tuned thanks to
the following environment variables.

\section ConfiguringWorkers Configuring Workers

<dl>

<dt>STARPU_NCPU</dt>
<dd>
\anchor STARPU_NCPU
\addindex __env__STARPU_NCPU
Specify the number of CPU workers (thus not including workers
dedicated to control accelerators). Note that by default, StarPU will
not allocate more CPU workers than there are physical CPUs, and that
some CPUs are used to control the accelerators.
</dd>

<dt>STARPU_NCPUS</dt>
<dd>
\anchor STARPU_NCPUS
\addindex __env__STARPU_NCPUS
This variable is deprecated. You should use \ref STARPU_NCPU.
</dd>

<dt>STARPU_NCUDA</dt>
<dd>
\anchor STARPU_NCUDA
\addindex __env__STARPU_NCUDA
Specify the number of CUDA devices that StarPU can use. If
\ref STARPU_NCUDA is lower than the number of physical devices, it is
possible to select which CUDA devices should be used by the means of the
environment variable \ref STARPU_WORKERS_CUDAID. By default, StarPU will
create as many CUDA workers as there are CUDA devices.
</dd>

<dt>STARPU_NOPENCL</dt>
<dd>
\anchor STARPU_NOPENCL
\addindex __env__STARPU_NOPENCL
OpenCL equivalent of the environment variable \ref STARPU_NCUDA.
</dd>

<dt>STARPU_OPENCL_ON_CPUS</dt>
<dd>
\anchor STARPU_OPENCL_ON_CPUS
\addindex __env__STARPU_OPENCL_ON_CPUS
By default, the OpenCL driver only enables GPU and accelerator
devices. By setting the environment variable \ref
STARPU_OPENCL_ON_CPUS to 1, the OpenCL driver will also enable CPU
devices.
</dd>

<dt>STARPU_OPENCL_ONLY_ON_CPUS</dt>
<dd>
\anchor STARPU_OPENCL_ONLY_ON_CPUS
\addindex __env__STARPU_OPENCL_ONLY_ON_CPUS
By default, the OpenCL driver enables GPU and accelerator
devices. By setting the environment variable \ref
STARPU_OPENCL_ONLY_ON_CPUS to 1, the OpenCL driver will ONLY enable
CPU devices.
</dd>

<dt>STARPU_NMIC</dt>
<dd>
\anchor STARPU_NMIC
\addindex __env__STARPU_NMIC
MIC equivalent of the environment variable \ref STARPU_NCUDA.
</dd>

<dt>STARPU_NSCC</dt>
<dd>
\anchor STARPU_NSCC
\addindex __env__STARPU_NSCC
SCC equivalent of the environment variable \ref STARPU_NCUDA.
</dd>

<dt>STARPU_WORKERS_NOBIND</dt>
<dd>
\anchor STARPU_WORKERS_NOBIND
\addindex __env__STARPU_WORKERS_NOBIND
Setting it to non-zero will prevent StarPU from binding its threads to
CPUs. This is for instance useful when running the testsuite in parallel.
</dd>

<dt>STARPU_WORKERS_CPUID</dt>
<dd>
\anchor STARPU_WORKERS_CPUID
\addindex __env__STARPU_WORKERS_CPUID
Passing an array of integers (starting from 0) in \ref STARPU_WORKERS_CPUID
specifies on which logical CPU the different workers should be
bound. For instance, if <c>STARPU_WORKERS_CPUID = "0 1 4 5"</c>, the first
worker will be bound to logical CPU #0, the second CPU worker will be bound to
logical CPU #1 and so on.  Note that the logical ordering of the CPUs is either
determined by the OS, or provided by the library <c>hwloc</c> in case it is
available.

Note that the first workers correspond to the CUDA workers, then come the
OpenCL workers, and finally the CPU workers. For example if
we have <c>STARPU_NCUDA=1</c>, <c>STARPU_NOPENCL=1</c>, <c>STARPU_NCPU=2</c>
and <c>STARPU_WORKERS_CPUID = "0 2 1 3"</c>, the CUDA device will be controlled
by logical CPU #0, the OpenCL device will be controlled by logical CPU #2, and
the logical CPUs #1 and #3 will be used by the CPU workers.

If the number of workers is larger than the array given in \ref
STARPU_WORKERS_CPUID, the workers are bound to the logical CPUs in a
round-robin fashion: if <c>STARPU_WORKERS_CPUID = "0 1"</c>, the first
and the third (resp. second and fourth) workers will be put on CPU #0
(resp. CPU #1).

This variable is ignored if the field
starpu_conf::use_explicit_workers_bindid passed to starpu_init() is
set.

</dd>

<dt>STARPU_WORKERS_CUDAID</dt>
<dd>
\anchor STARPU_WORKERS_CUDAID
\addindex __env__STARPU_WORKERS_CUDAID
Similarly to the \ref STARPU_WORKERS_CPUID environment variable, it is
possible to select which CUDA devices should be used by StarPU. On a machine
equipped with 4 GPUs, setting <c>STARPU_WORKERS_CUDAID = "1 3"</c> and
<c>STARPU_NCUDA=2</c> specifies that 2 CUDA workers should be created, and that
they should use CUDA devices #1 and #3 (the logical ordering of the devices is
the one reported by CUDA).

This variable is ignored if the field
starpu_conf::use_explicit_workers_cuda_gpuid passed to starpu_init()
is set.
</dd>

<dt>STARPU_WORKERS_OPENCLID</dt>
<dd>
\anchor STARPU_WORKERS_OPENCLID
\addindex __env__STARPU_WORKERS_OPENCLID
OpenCL equivalent of the \ref STARPU_WORKERS_CUDAID environment variable.

This variable is ignored if the field
starpu_conf::use_explicit_workers_opencl_gpuid passed to starpu_init()
is set.
</dd>

<dt>STARPU_WORKERS_MICID</dt>
<dd>
\anchor STARPU_WORKERS_MICID
\addindex __env__STARPU_WORKERS_MICID
MIC equivalent of the \ref STARPU_WORKERS_CUDAID environment variable.

This variable is ignored if the field
starpu_conf::use_explicit_workers_mic_deviceid passed to starpu_init()
is set.
</dd>

<dt>STARPU_WORKERS_SCCID</dt>
<dd>
\anchor STARPU_WORKERS_SCCID
\addindex __env__STARPU_WORKERS_SCCID
SCC equivalent of the \ref STARPU_WORKERS_CUDAID environment variable.

This variable is ignored if the field
starpu_conf::use_explicit_workers_scc_deviceid passed to starpu_init()
is set.
</dd>

<dt>STARPU_SINGLE_COMBINED_WORKER</dt>
<dd>
\anchor STARPU_SINGLE_COMBINED_WORKER
\addindex __env__STARPU_SINGLE_COMBINED_WORKER
If set, StarPU will create several workers which won't be able to work
concurrently. It will by default create combined workers which size goes from 1
to the total number of CPU workers in the system. \ref STARPU_MIN_WORKERSIZE
and \ref STARPU_MAX_WORKERSIZE can be used to change this default.
</dd>

<dt>STARPU_MIN_WORKERSIZE</dt>
<dd>
\anchor STARPU_MIN_WORKERSIZE
\addindex __env__STARPU_MIN_WORKERSIZE
\ref STARPU_MIN_WORKERSIZE
permits to specify the minimum size of the combined workers (instead of the default 2)
</dd>

<dt>STARPU_MAX_WORKERSIZE</dt>
<dd>
\anchor STARPU_MAX_WORKERSIZE
\addindex __env__STARPU_MAX_WORKERSIZE
\ref STARPU_MAX_WORKERSIZE
permits to specify the minimum size of the combined workers (instead of the
number of CPU workers in the system)
</dd>

<dt>STARPU_SYNTHESIZE_ARITY_COMBINED_WORKER</dt>
<dd>
\anchor STARPU_SYNTHESIZE_ARITY_COMBINED_WORKER
\addindex __env__STARPU_SYNTHESIZE_ARITY_COMBINED_WORKER
Let the user decide how many elements are allowed between combined workers
created from hwloc information. For instance, in the case of sockets with 6
cores without shared L2 caches, if \ref STARPU_SYNTHESIZE_ARITY_COMBINED_WORKER is
set to 6, no combined worker will be synthesized beyond one for the socket
and one per core. If it is set to 3, 3 intermediate combined workers will be
synthesized, to divide the socket cores into 3 chunks of 2 cores. If it set to
2, 2 intermediate combined workers will be synthesized, to divide the the socket
cores into 2 chunks of 3 cores, and then 3 additional combined workers will be
synthesized, to divide the former synthesized workers into a bunch of 2 cores,
and the remaining core (for which no combined worker is synthesized since there
is already a normal worker for it).

The default, 2, thus makes StarPU tend to building a binary trees of combined
workers.
</dd>

<dt>STARPU_DISABLE_ASYNCHRONOUS_COPY</dt>
<dd>
\anchor STARPU_DISABLE_ASYNCHRONOUS_COPY
\addindex __env__STARPU_DISABLE_ASYNCHRONOUS_COPY
Disable asynchronous copies between CPU and GPU devices.
The AMD implementation of OpenCL is known to
fail when copying data asynchronously. When using this implementation,
it is therefore necessary to disable asynchronous data transfers.
</dd>

<dt>STARPU_DISABLE_ASYNCHRONOUS_CUDA_COPY</dt>
<dd>
\anchor STARPU_DISABLE_ASYNCHRONOUS_CUDA_COPY
\addindex __env__STARPU_DISABLE_ASYNCHRONOUS_CUDA_COPY
Disable asynchronous copies between CPU and CUDA devices.
</dd>

<dt>STARPU_DISABLE_ASYNCHRONOUS_OPENCL_COPY</dt>
<dd>
\anchor STARPU_DISABLE_ASYNCHRONOUS_OPENCL_COPY
\addindex __env__STARPU_DISABLE_ASYNCHRONOUS_OPENCL_COPY
Disable asynchronous copies between CPU and OpenCL devices.
The AMD implementation of OpenCL is known to
fail when copying data asynchronously. When using this implementation,
it is therefore necessary to disable asynchronous data transfers.
</dd>

<dt>STARPU_DISABLE_ASYNCHRONOUS_MIC_COPY</dt>
<dd>
\anchor STARPU_DISABLE_ASYNCHRONOUS_MIC_COPY
\addindex __env__STARPU_DISABLE_ASYNCHRONOUS_MIC_COPY
Disable asynchronous copies between CPU and MIC devices.
</dd>

<dt>STARPU_ENABLE_CUDA_GPU_GPU_DIRECT</dt>
<dd>
\anchor STARPU_ENABLE_CUDA_GPU_GPU_DIRECT
\addindex __env__STARPU_ENABLE_CUDA_GPU_GPU_DIRECT
Enable (1) or Disable (0) direct CUDA transfers from GPU to GPU, without copying
through RAM. The default is Enabled.
This permits to test the performance effect of GPU-Direct.
</dd>

</dl>

\section ConfiguringTheSchedulingEngine Configuring The Scheduling Engine

<dl>

<dt>STARPU_SCHED</dt>
<dd>
\anchor STARPU_SCHED
\addindex __env__STARPU_SCHED
Choose between the different scheduling policies proposed by StarPU: work
random, stealing, greedy, with performance models, etc.

Use <c>STARPU_SCHED=help</c> to get the list of available schedulers.
</dd>

<dt>STARPU_CALIBRATE</dt>
<dd>
\anchor STARPU_CALIBRATE
\addindex __env__STARPU_CALIBRATE
If this variable is set to 1, the performance models are calibrated during
the execution. If it is set to 2, the previous values are dropped to restart
calibration from scratch. Setting this variable to 0 disable calibration, this
is the default behaviour.

Note: this currently only applies to <c>dm</c> and <c>dmda</c> scheduling policies.
</dd>

<dt>STARPU_BUS_CALIBRATE</dt>
<dd>
\anchor STARPU_BUS_CALIBRATE
\addindex __env__STARPU_BUS_CALIBRATE
If this variable is set to 1, the bus is recalibrated during intialization.
</dd>

<dt>STARPU_PREFETCH</dt>
<dd>
\anchor STARPU_PREFETCH
\addindex __env__STARPU_PREFETCH
This variable indicates whether data prefetching should be enabled (0 means
that it is disabled). If prefetching is enabled, when a task is scheduled to be
executed e.g. on a GPU, StarPU will request an asynchronous transfer in
advance, so that data is already present on the GPU when the task starts. As a
result, computation and data transfers are overlapped.
Note that prefetching is enabled by default in StarPU.
</dd>

<dt>STARPU_SCHED_ALPHA</dt>
<dd>
\anchor STARPU_SCHED_ALPHA
\addindex __env__STARPU_SCHED_ALPHA
To estimate the cost of a task StarPU takes into account the estimated
computation time (obtained thanks to performance models). The alpha factor is
the coefficient to be applied to it before adding it to the communication part.
</dd>

<dt>STARPU_SCHED_BETA</dt>
<dd>
\anchor STARPU_SCHED_BETA
\addindex __env__STARPU_SCHED_BETA
To estimate the cost of a task StarPU takes into account the estimated
data transfer time (obtained thanks to performance models). The beta factor is
the coefficient to be applied to it before adding it to the computation part.
</dd>

<dt>STARPU_SCHED_GAMMA</dt>
<dd>
\anchor STARPU_SCHED_GAMMA
\addindex __env__STARPU_SCHED_GAMMA
Define the execution time penalty of a joule (\ref Power-basedScheduling).
</dd>

<dt>STARPU_IDLE_POWER</dt>
<dd>
\anchor STARPU_IDLE_POWER
\addindex __env__STARPU_IDLE_POWER
Define the idle power of the machine (\ref Power-basedScheduling).
</dd>

<dt>STARPU_PROFILING</dt>
<dd>
\anchor STARPU_PROFILING
\addindex __env__STARPU_PROFILING
Enable on-line performance monitoring (\ref EnablingOn-linePerformanceMonitoring).
</dd>

</dl>

\section Extensions Extensions

<dl>

<dt>SOCL_OCL_LIB_OPENCL</dt>
<dd>
\anchor SOCL_OCL_LIB_OPENCL
\addindex __env__SOCL_OCL_LIB_OPENCL
THE SOCL test suite is only run when the environment variable \ref
SOCL_OCL_LIB_OPENCL is defined. It should contain the location
of the file <c>libOpenCL.so</c> of the OCL ICD implementation.
</dd>

<dt>OCL_ICD_VENDORS</dt>
<dd>
\anchor OCL_ICD_VENDORS
\addindex __env__OCL_ICD_VENDORS
When using SOCL with OpenCL ICD
(https://forge.imag.fr/projects/ocl-icd/), this variable may be used
to point to the directory where ICD files are installed. The default
directory is <c>/etc/OpenCL/vendors</c>. StarPU installs ICD
files in the directory <c>$prefix/share/starpu/opencl/vendors</c>.
</dd>

<dt>STARPU_COMM_STATS</dt>
<dd>
\anchor STARPU_COMM_STATS
\addindex __env__STARPU_COMM_STATS
Communication statistics for starpumpi (\ref MPISupport)
will be enabled when the environment variable \ref STARPU_COMM_STATS
is defined to an value other than 0.
</dd>

<dt>STARPU_MPI_CACHE</dt>
<dd>
\anchor STARPU_MPI_CACHE
\addindex __env__STARPU_MPI_CACHE
Communication cache for starpumpi (\ref MPISupport) will be
disabled when the environment variable \ref STARPU_MPI_CACHE is set
to 0. It is enabled by default or for any other values of the variable
\ref STARPU_MPI_CACHE.
</dd>

</dl>

\section MiscellaneousAndDebug Miscellaneous And Debug

<dl>

<dt>STARPU_HOME</dt>
<dd>
\anchor STARPU_HOME
\addindex __env__STARPU_HOME
This specifies the main directory in which StarPU stores its
configuration files. The default is <c>$HOME</c> on Unix environments,
and <c>$USERPROFILE</c> on Windows environments.
</dd>

<dt>STARPU_HOSTNAME</dt>
<dd>
\anchor STARPU_HOSTNAME
\addindex __env__STARPU_HOSTNAME
When set, force the hostname to be used when dealing performance model
files. Models are indexed by machine name. When running for example on
a homogenenous cluster, it is possible to share the models between
machines by setting <c>export STARPU_HOSTNAME=some_global_name</c>.
</dd>

<dt>STARPU_OPENCL_PROGRAM_DIR</dt>
<dd>
\anchor STARPU_OPENCL_PROGRAM_DIR
\addindex __env__STARPU_OPENCL_PROGRAM_DIR
This specifies the directory where the OpenCL codelet source files are
located. The function starpu_opencl_load_program_source() looks
for the codelet in the current directory, in the directory specified
by the environment variable \ref STARPU_OPENCL_PROGRAM_DIR, in the
directory <c>share/starpu/opencl</c> of the installation directory of
StarPU, and finally in the source directory of StarPU.
</dd>

<dt>STARPU_SILENT</dt>
<dd>
\anchor STARPU_SILENT
\addindex __env__STARPU_SILENT
This variable allows to disable verbose mode at runtime when StarPU
has been configured with the option \ref enable-verbose "--enable-verbose". It also
disables the display of StarPU information and warning messages.
</dd>

<dt>STARPU_LOGFILENAME</dt>
<dd>
\anchor STARPU_LOGFILENAME
\addindex __env__STARPU_LOGFILENAME
This variable specifies in which file the debugging output should be saved to.
</dd>

<dt>STARPU_FXT_PREFIX</dt>
<dd>
\anchor STARPU_FXT_PREFIX
\addindex __env__STARPU_FXT_PREFIX
This variable specifies in which directory to save the trace generated if FxT is enabled. It needs to have a trailing '/' character.
</dd>

<dt>STARPU_LIMIT_CUDA_devid_MEM</dt>
<dd>
\anchor STARPU_LIMIT_CUDA_devid_MEM
\addindex __env__STARPU_LIMIT_CUDA_devid_MEM
This variable specifies the maximum number of megabytes that should be
available to the application on the CUDA device with the identifier
<c>devid</c>. This variable is intended to be used for experimental
purposes as it emulates devices that have a limited amount of memory.
When defined, the variable overwrites the value of the variable
\ref STARPU_LIMIT_CUDA_MEM.
</dd>

<dt>STARPU_LIMIT_CUDA_MEM</dt>
<dd>
\anchor STARPU_LIMIT_CUDA_MEM
\addindex __env__STARPU_LIMIT_CUDA_MEM
This variable specifies the maximum number of megabytes that should be
available to the application on each CUDA devices. This variable is
intended to be used for experimental purposes as it emulates devices
that have a limited amount of memory.
</dd>

<dt>STARPU_LIMIT_OPENCL_devid_MEM</dt>
<dd>
\anchor STARPU_LIMIT_OPENCL_devid_MEM
\addindex __env__STARPU_LIMIT_OPENCL_devid_MEM
This variable specifies the maximum number of megabytes that should be
available to the application on the OpenCL device with the identifier
<c>devid</c>. This variable is intended to be used for experimental
purposes as it emulates devices that have a limited amount of memory.
When defined, the variable overwrites the value of the variable
\ref STARPU_LIMIT_OPENCL_MEM.
</dd>

<dt>STARPU_LIMIT_OPENCL_MEM</dt>
<dd>
\anchor STARPU_LIMIT_OPENCL_MEM
\addindex __env__STARPU_LIMIT_OPENCL_MEM
This variable specifies the maximum number of megabytes that should be
available to the application on each OpenCL devices. This variable is
intended to be used for experimental purposes as it emulates devices
that have a limited amount of memory.
</dd>

<dt>STARPU_LIMIT_CPU_MEM</dt>
<dd>
\anchor STARPU_LIMIT_CPU_MEM
\addindex __env__STARPU_LIMIT_CPU_MEM
This variable specifies the maximum number of megabytes that should be
available to the application on each CPU device. This variable is
intended to be used for experimental purposes as it emulates devices
that have a limited amount of memory.
</dd>

<dt>STARPU_GENERATE_TRACE</dt>
<dd>
\anchor STARPU_GENERATE_TRACE
\addindex __env__STARPU_GENERATE_TRACE
When set to <c>1</c>, this variable indicates that StarPU should automatically
generate a Paje trace when starpu_shutdown() is called.
</dd>

<dt>STARPU_MEMORY_STATS</dt>
<dd>
\anchor STARPU_MEMORY_STATS
\addindex __env__STARPU_MEMORY_STATS
When set to 0, disable the display of memory statistics on data which
have not been unregistered at the end of the execution (\ref MemoryFeedback).
</dd>

<dt>STARPU_BUS_STATS</dt>
<dd>
\anchor STARPU_BUS_STATS
\addindex __env__STARPU_BUS_STATS
When defined, statistics about data transfers will be displayed when calling
starpu_shutdown() (\ref Profiling).
</dd>

<dt>STARPU_WORKER_STATS</dt>
<dd>
\anchor STARPU_WORKER_STATS
\addindex __env__STARPU_WORKER_STATS
When defined, statistics about the workers will be displayed when calling
starpu_shutdown() (\ref Profiling). When combined with the
environment variable \ref STARPU_PROFILING, it displays the power
consumption (\ref Power-basedScheduling).
</dd>

<dt>STARPU_STATS</dt>
<dd>
\anchor STARPU_STATS
\addindex __env__STARPU_STATS
When set to 0, data statistics will not be displayed at the
end of the execution of an application (\ref DataStatistics).
</dd>

<dt>STARPU_WATCHDOG_TIMEOUT</dt>
<dd>
\anchor STARPU_WATCHDOG_TIMEOUT
\addindex __env__STARPU_WATCHDOG_TIMEOUT
When set to a value other than 0, allows to make StarPU print an error
message whenever StarPU does not terminate any task for 10ms. Should
be used in combination with \ref STARPU_WATCHDOG_CRASH (see \ref
DetectionStuckConditions).
</dd>

<dt>STARPU_WATCHDOG_CRASH</dt>
<dd>
\anchor STARPU_WATCHDOG_CRASH
\addindex __env__STARPU_WATCHDOG_CRASH
When set to a value other than 0, it triggers a crash when the watch
dog is reached, thus allowing to catch the situation in gdb, etc
(see \ref DetectionStuckConditions)
</dd>

<dt>STARPU_HISTORY_MAX_ERROR</dt>
<dd>
\anchor STARPU_HISTORY_MAX_ERROR
\addindex __env__STARPU_HISTORY_MAX_ERROR
History-based performance models will drop measurements which are really far
froom the measured average. This specifies the allowed variation. The default is
10, i.e. the measurement is allowed to be 110% faster or 110% slower than the
average.
</dd>

</dl>

\section ConfiguringTheHypervisor Configuring The Hypervisor

<dl>

<dt>SC_HYPERVISOR_POLICY</dt>
<dd>
\anchor SC_HYPERVISOR_POLICY
\addindex __env__SC_HYPERVISOR_POLICY
Choose between the different resizing policies proposed by StarPU for the hypervisor: 
idle, app_driven, feft_lp, teft_lp; ispeed_lp, throughput_lp etc.

Use <c>SC_HYPERVISOR_POLICY=help</c> to get the list of available policies for the hypervisor
</dd>

<dt>SC_HYPERVISOR_TRIGGER_RESIZE</dt>
<dd>
\anchor SC_HYPERVISOR_TRIGGER_RESIZE
\addindex __env__SC_HYPERVISOR_TRIGGER_RESIZE
Choose how should the hypervisor be triggered: <c>speed</c> if the resizing algorithm should
be called whenever the speed of the context does not correspond to an optimal precomputed value,
<c>idle</c> it the resizing algorithm should be called whenever the workers are idle for a period
longer than the value indicated when configuring the hypervisor.
</dd>

<dt>SC_HYPERVISOR_START_RESIZE</dt>
<dd>
\anchor SC_HYPERVISOR_START_RESIZE
\addindex __env__SC_HYPERVISOR_START_RESIZE
Indicate the moment when the resizing should be available. The value correspond to the percentage
of the total time of execution of the application. The default value is the resizing frame.
</dd>

<dt>SC_HYPERVISOR_MAX_SPEED_GAP</dt>
<dd>
\anchor SC_HYPERVISOR_MAX_SPEED_GAP
\addindex __env__SC_HYPERVISOR_MAX_SPEED_GAP
Indicate the ratio of speed difference between contexts that should trigger the hypervisor.
This situation may occur only when a theoretical speed could not be computed and the hypervisor
has no value to compare the speed to. Otherwise the resizing of a context is not influenced by the 
the speed of the other contexts, but only by the the value that a context should have.
</dd>

<dt>SC_HYPERVISOR_STOP_PRINT</dt>
<dd>
\anchor SC_HYPERVISOR_STOP_PRINT
\addindex __env__SC_HYPERVISOR_STOP_PRINT
By default the values of the speed of the workers is printed during the execution
of the application. If the value 1 is given to this environment variable this printing
is not done.
</dd>

<dt>SC_HYPERVISOR_LAZY_RESIZE</dt>
<dd>
\anchor SC_HYPERVISOR_LAZY_RESIZE
\addindex __env__SC_HYPERVISOR_LAZY_RESIZE
By default the hypervisor resizes the contexts in a lazy way, that is workers are firstly added to a new context
before removing them from the previous one. Once this workers are clearly taken into account 
into the new context (a task was poped there) we remove them from the previous one. However if the application
would like that the change in the distribution of workers should change right away this variable should be set to 0
</dd>

<dt>SC_HYPERVISOR_SAMPLE_CRITERIA</dt>
<dd>
\anchor SC_HYPERVISOR_SAMPLE_CRITERIA
\addindex __env__SC_HYPERVISOR_SAMPLE_CRITERIA
By default the hypervisor uses a sample of flops when computing the speed of the contexts and of the workers.
If this variable is set to <c>time</c> the hypervisor uses a sample of time (10% of an aproximation of the total
execution time of the application)
</dd>

</dl>

*/