starpu-max/doc/doxygen/chapters/370_online_performance_tools.doxy

/* StarPU --- Runtime system for heterogeneous multicore architectures.
 *
 * Copyright (C) 2011,2012,2016                           Inria
 * Copyright (C) 2010-2019                                CNRS
 * Copyright (C) 2009-2011,2014,2016,2018-2019            Université de Bordeaux
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 * StarPU is free software; you can redistribute it and/or modify
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/*! \page OnlinePerformanceTools Online Performance Tools

\section On-linePerformanceFeedback On-line Performance Feedback

\subsection EnablingOn-linePerformanceMonitoring Enabling On-line Performance Monitoring

In order to enable online performance monitoring, the application can
call starpu_profiling_status_set() with the parameter
::STARPU_PROFILING_ENABLE. It is possible to detect whether monitoring
is already enabled or not by calling starpu_profiling_status_get().
Enabling monitoring also reinitialize all previously collected
feedback. The environment variable \ref STARPU_PROFILING can also be
set to <c>1</c> to achieve the same effect. The function
starpu_profiling_init() can also be called during the execution to
reinitialize performance counters and to start the profiling if the
environment variable \ref STARPU_PROFILING is set to <c>1</c>.

Likewise, performance monitoring is stopped by calling
starpu_profiling_status_set() with the parameter
::STARPU_PROFILING_DISABLE. Note that this does not reset the
performance counters so that the application may consult them later
on.

More details about the performance monitoring API are available in \ref API_Profiling.

\subsection Per-taskFeedback Per-task Feedback

If profiling is enabled, a pointer to a structure
starpu_profiling_task_info is put in the field
starpu_task::profiling_info when a task terminates. This structure is
automatically destroyed when the task structure is destroyed, either
automatically or by calling starpu_task_destroy().

The structure starpu_profiling_task_info indicates the date when the
task was submitted (starpu_profiling_task_info::submit_time), started
(starpu_profiling_task_info::start_time), and terminated
(starpu_profiling_task_info::end_time), relative to the initialization
of StarPU with starpu_init(). It also specifies the identifier of the worker
that has executed the task (starpu_profiling_task_info::workerid).
These date are stored as <c>timespec</c> structures which the user may convert
into micro-seconds using the helper function
starpu_timing_timespec_to_us().

It it worth noting that the application may directly access this structure from
the callback executed at the end of the task. The structure starpu_task
associated to the callback currently being executed is indeed accessible with
the function starpu_task_get_current().

\subsection Per-codeletFeedback Per-codelet Feedback

The field starpu_codelet::per_worker_stats is
an array of counters. The <c>i</c>-th entry of the array is incremented every time a
task implementing the codelet is executed on the <c>i</c>-th worker.
This array is not reinitialized when profiling is enabled or disabled.

\subsection Per-workerFeedback Per-worker Feedback

The second argument returned by the function
starpu_profiling_worker_get_info() is a structure
starpu_profiling_worker_info that gives statistics about the specified
worker. This structure specifies when StarPU started collecting
profiling information for that worker
(starpu_profiling_worker_info::start_time), the
duration of the profiling measurement interval
(starpu_profiling_worker_info::total_time), the time spent executing
kernels (starpu_profiling_worker_info::executing_time), the time
spent sleeping because there is no task to execute at all
(starpu_profiling_worker_info::sleeping_time), and the number of tasks that were executed
while profiling was enabled. These values give an estimation of the
proportion of time spent do real work, and the time spent either
sleeping because there are not enough executable tasks or simply
wasted in pure StarPU overhead.

Calling starpu_profiling_worker_get_info() resets the profiling
information associated to a worker.

To easily display all this information, the environment variable
\ref STARPU_WORKER_STATS can be set to <c>1</c> (in addition to setting
\ref STARPU_PROFILING to 1). A summary will then be displayed at program termination:

\verbatim
Worker stats:
CUDA 0.0 (4.7 GiB)
	480 task(s)
	total: 1574.82 ms executing: 1510.72 ms sleeping: 0.00 ms overhead 64.10 ms
	325.217970 GFlop/s

CPU 0
	22 task(s)
	total: 1574.82 ms executing: 1364.81 ms sleeping: 0.00 ms overhead 210.01 ms
	7.512057 GFlop/s

CPU 1
	14 task(s)
	total: 1574.82 ms executing: 1500.13 ms sleeping: 0.00 ms overhead 74.69 ms
	6.675853 GFlop/s

CPU 2
	14 task(s)
	total: 1574.82 ms executing: 1553.12 ms sleeping: 0.00 ms overhead 21.70 ms
	7.152886 GFlop/s
\endverbatim

The number of GFlops is available because the starpu_task::flops field of the
tasks were filled (or \ref STARPU_FLOPS used in starpu_task_insert()).

When an FxT trace is generated (see \ref GeneratingTracesWithFxT), it is also
possible to use the tool <c>starpu_workers_activity</c> (see
\ref MonitoringActivity) to generate a graphic showing the evolution of
these values during the time, for the different workers.

\subsection Bus-relatedFeedback Bus-related Feedback

// how to enable/disable performance monitoring
// what kind of information do we get ?

The bus speed measured by StarPU can be displayed by using the tool
<c>starpu_machine_display</c>, for instance:

\verbatim
StarPU has found:
        3 CUDA devices
                CUDA 0 (Tesla C2050 02:00.0)
                CUDA 1 (Tesla C2050 03:00.0)
                CUDA 2 (Tesla C2050 84:00.0)
from    to RAM          to CUDA 0       to CUDA 1       to CUDA 2
RAM     0.000000        5176.530428     5176.492994     5191.710722
CUDA 0  4523.732446     0.000000        2414.074751     2417.379201
CUDA 1  4523.718152     2414.078822     0.000000        2417.375119
CUDA 2  4534.229519     2417.069025     2417.060863     0.000000
\endverbatim

Statistics about the data transfers which were performed and temporal average
of bandwidth usage can be obtained by setting the environment variable
\ref STARPU_BUS_STATS to <c>1</c>; a summary will then be displayed at program termination:

\verbatim
Data transfer stats:
	RAM 0 -> CUDA 0	319.92 MB	213.10 MB/s	(transfers : 91 - avg 3.52 MB)
	CUDA 0 -> RAM 0	214.45 MB	142.85 MB/s	(transfers : 61 - avg 3.52 MB)
	RAM 0 -> CUDA 1	302.34 MB	201.39 MB/s	(transfers : 86 - avg 3.52 MB)
	CUDA 1 -> RAM 0	133.59 MB	88.99 MB/s	(transfers : 38 - avg 3.52 MB)
	CUDA 0 -> CUDA 1	144.14 MB	96.01 MB/s	(transfers : 41 - avg 3.52 MB)
	CUDA 1 -> CUDA 0	130.08 MB	86.64 MB/s	(transfers : 37 - avg 3.52 MB)
	RAM 0 -> CUDA 2	312.89 MB	208.42 MB/s	(transfers : 89 - avg 3.52 MB)
	CUDA 2 -> RAM 0	133.59 MB	88.99 MB/s	(transfers : 38 - avg 3.52 MB)
	CUDA 0 -> CUDA 2	151.17 MB	100.69 MB/s	(transfers : 43 - avg 3.52 MB)
	CUDA 2 -> CUDA 0	105.47 MB	70.25 MB/s	(transfers : 30 - avg 3.52 MB)
	CUDA 1 -> CUDA 2	175.78 MB	117.09 MB/s	(transfers : 50 - avg 3.52 MB)
	CUDA 2 -> CUDA 1	203.91 MB	135.82 MB/s	(transfers : 58 - avg 3.52 MB)
Total transfers: 2.27 GB
\endverbatim

\subsection MPI-relatedFeedback MPI-related Feedback

Statistics about the data transfers which were performed over MPI can be
obtained by setting the environment variable \ref STARPU_COMM_STATS to <c>1</c>;
a summary will then be displayed at program termination:

\verbatim
[starpu_comm_stats][1] TOTAL:	456.000000 B	0.000435 MB	 0.000188 B/s	 0.000000 MB/s
[starpu_comm_stats][1:0]	456.000000 B	0.000435 MB	 0.000188 B/s	 0.000000 MB/s

[starpu_comm_stats][0] TOTAL:	456.000000 B	0.000435 MB	 0.000188 B/s	 0.000000 MB/s
[starpu_comm_stats][0:1]	456.000000 B	0.000435 MB	 0.000188 B/s	 0.000000 MB/s
\endverbatim

These statistics can be plotted as heatmaps using StarPU tool <c>starpu_mpi_comm_matrix.py</c> (see \ref MPIDebug).

\section TaskAndWorkerProfiling Task And Worker Profiling

A full example showing how to use the profiling API is available in
the StarPU sources in the directory <c>examples/profiling/</c>.

\code{.c}
struct starpu_task *task = starpu_task_create();
task->cl = &cl;
task->synchronous = 1;
/* We will destroy the task structure by hand so that we can
 * query the profiling info before the task is destroyed. */
task->destroy = 0;

/* Submit and wait for completion (since synchronous was set to 1) */
starpu_task_submit(task);

/* The task is finished, get profiling information */
struct starpu_profiling_task_info *info = task->profiling_info;

/* How much time did it take before the task started ? */
double delay += starpu_timing_timespec_delay_us(&info->submit_time, &info->start_time);

/* How long was the task execution ? */
double length += starpu_timing_timespec_delay_us(&info->start_time, &info->end_time);

/* We no longer need the task structure */
starpu_task_destroy(task);
\endcode

\code{.c}
/* Display the occupancy of all workers during the test */
int worker;
for (worker = 0; worker < starpu_worker_get_count(); worker++)
{
        struct starpu_profiling_worker_info worker_info;
        int ret = starpu_profiling_worker_get_info(worker, &worker_info);
        STARPU_ASSERT(!ret);

        double total_time = starpu_timing_timespec_to_us(&worker_info.total_time);
        double executing_time = starpu_timing_timespec_to_us(&worker_info.executing_time);
        double sleeping_time = starpu_timing_timespec_to_us(&worker_info.sleeping_time);
        double overhead_time = total_time - executing_time - sleeping_time;

        float executing_ratio = 100.0*executing_time/total_time;
        float sleeping_ratio = 100.0*sleeping_time/total_time;
        float overhead_ratio = 100.0 - executing_ratio - sleeping_ratio;

        char workername[128];
        starpu_worker_get_name(worker, workername, 128);
        fprintf(stderr, "Worker %s:\n", workername);
        fprintf(stderr, "\ttotal time: %.2lf ms\n", total_time*1e-3);
        fprintf(stderr, "\texec time: %.2lf ms (%.2f %%)\n", executing_time*1e-3, executing_ratio);
        fprintf(stderr, "\tblocked time: %.2lf ms (%.2f %%)\n", sleeping_time*1e-3, sleeping_ratio);
        fprintf(stderr, "\toverhead time: %.2lf ms (%.2f %%)\n", overhead_time*1e-3, overhead_ratio);
}
\endcode

\section PerformanceModelExample Performance Model Example

To achieve good scheduling, StarPU scheduling policies need to be able to
estimate in advance the duration of a task. This is done by giving to codelets
a performance model, by defining a structure starpu_perfmodel and
providing its address in the field starpu_codelet::model. The fields
starpu_perfmodel::symbol and starpu_perfmodel::type are mandatory, to
give a name to the model, and the type of the model, since there are
several kinds of performance models. For compatibility, make sure to
initialize the whole structure to zero, either by using explicit
memset(), or by letting the compiler implicitly do it as examplified
below.

<ul>
<li>
Measured at runtime (model type ::STARPU_HISTORY_BASED). This assumes that for a
given set of data input/output sizes, the performance will always be about the
same. This is very true for regular kernels on GPUs for instance (<0.1% error),
and just a bit less true on CPUs (~=1% error). This also assumes that there are
few different sets of data input/output sizes. StarPU will then keep record of
the average time of previous executions on the various processing units, and use
it as an estimation. History is done per task size, by using a hash of the input
and ouput sizes as an index.
It will also save it in <c>$STARPU_HOME/.starpu/sampling/codelets</c>
for further executions, and can be observed by using the tool
<c>starpu_perfmodel_display</c>, or drawn by using
the tool <c>starpu_perfmodel_plot</c> (\ref PerformanceModelCalibration).  The
models are indexed by machine name. To
share the models between machines (e.g. for a homogeneous cluster), use
<c>export STARPU_HOSTNAME=some_global_name</c>. Measurements are only done
when using a task scheduler which makes use of it, such as
<c>dmda</c>. Measurements can also be provided explicitly by the application, by
using the function starpu_perfmodel_update_history().

The following is a small code example.

If e.g. the code is recompiled with other compilation options, or several
variants of the code are used, the <c>symbol</c> string should be changed to reflect
that, in order to recalibrate a new model from zero. The <c>symbol</c> string can even
be constructed dynamically at execution time, as long as this is done before
submitting any task using it.

\code{.c}
static struct starpu_perfmodel mult_perf_model =
{
    .type = STARPU_HISTORY_BASED,
    .symbol = "mult_perf_model"
};

struct starpu_codelet cl =
{
    .cpu_funcs = { cpu_mult },
    .cpu_funcs_name = { "cpu_mult" },
    .nbuffers = 3,
    .modes = { STARPU_R, STARPU_R, STARPU_W },
    /* for the scheduling policy to be able to use performance models */
    .model = &mult_perf_model
};
\endcode

</li>
<li>
Measured at runtime and refined by regression (model types
::STARPU_REGRESSION_BASED and ::STARPU_NL_REGRESSION_BASED). This
still assumes performance regularity, but works
with various data input sizes, by applying regression over observed
execution times. ::STARPU_REGRESSION_BASED uses an <c>a*n^b</c> regression
form, ::STARPU_NL_REGRESSION_BASED uses an <c>a*n^b+c</c> (more precise than
::STARPU_REGRESSION_BASED, but costs a lot more to compute).

For instance,
<c>tests/perfmodels/regression_based.c</c> uses a regression-based performance
model for the function memset().

Of course, the application has to issue
tasks with varying size so that the regression can be computed. StarPU will not
trust the regression unless there is at least 10% difference between the minimum
and maximum observed input size. It can be useful to set the
environment variable \ref STARPU_CALIBRATE to <c>1</c> and run the application
on varying input sizes with \ref STARPU_SCHED set to <c>dmda</c> scheduler,
so as to feed the performance model for a variety of
inputs. The application can also provide the measurements explictly by
using the function starpu_perfmodel_update_history(). The tools
<c>starpu_perfmodel_display</c> and <c>starpu_perfmodel_plot</c> can
be used to observe how much the performance model is calibrated
(\ref PerformanceModelCalibration); when their output look good,
\ref STARPU_CALIBRATE can be reset to <c>0</c> to let
StarPU use the resulting performance model without recording new measures, and
\ref STARPU_SCHED can be set to <c>dmda</c> to benefit from the performance models. If
the data input sizes vary a lot, it is really important to set
\ref STARPU_CALIBRATE to <c>0</c>, otherwise StarPU will continue adding the
measures, and result with a very big performance model, which will take time a
lot of time to load and save.

For non-linear regression, since computing it
is quite expensive, it is only done at termination of the application. This
means that the first execution of the application will use only history-based
performance model to perform scheduling, without using regression.
</li>

<li>

Another type of model is ::STARPU_MULTIPLE_REGRESSION_BASED, which
is based on multiple linear regression. In this model, the user
defines both the relevant parameters and the equation for computing the
task duration.


\f[
T_{kernel} = a + b(M^{\alpha_1} * N^{\beta_1} * K^{\gamma_1}) + c(M^{\alpha_2} * N^{\beta_2} * K^{\gamma_2}) + ...
\f]


\f$M, N, K\f$ are the parameters of the task, added at the task
creation. These need to be extracted by the <c>cl_perf_func</c>
function, which should be defined by the user. \f$\alpha, \beta,
\gamma\f$ are the exponents defined by the user in
<c>model->combinations</c> table. Finally, coefficients \f$a, b, c\f$
are computed automatically by the StarPU at the end of the execution, using least
squares method of the <c>dgels_</c> LAPACK function.

<c>examples/mlr/mlr.c</c> example provides more details on
the usage of ::STARPU_MULTIPLE_REGRESSION_BASED models.

Coefficients computation is done at the end of the execution, and the
results are stored in standard codelet perfmodel files. Additional
files containing the duration of task together with the value of each
parameter are stored in <c>.starpu/sampling/codelets/tmp/</c>
directory. These files are reused when \ref STARPU_CALIBRATE
environment variable is set to <c>1</c>, to recompute coefficients
based on the current, but also on the previous
executions. Additionally, when multiple linear regression models are
disabled (using \ref disable-mlr "--disable-mlr" configure option) or when the
<c>model->combinations</c> are not defined, StarPU will still write
output files into <c>.starpu/sampling/codelets/tmp/</c> to allow
performing an analysis. This analysis typically aims at finding the
most appropriate equation for the codelet and
<c>tools/starpu_mlr_analysis</c> script provides an example of how to
perform such study.

</li>

<li>
Provided as an estimation from the application itself (model type
::STARPU_COMMON and field starpu_perfmodel::cost_function),
see for instance
<c>examples/common/blas_model.h</c> and <c>examples/common/blas_model.c</c>.
</li>

<li>
Provided explicitly by the application (model type ::STARPU_PER_ARCH):
either field starpu_perfmodel::arch_cost_function, or
the fields <c>.per_arch[arch][nimpl].cost_function</c> have to be
filled with pointers to functions which return the expected duration
of the task in micro-seconds, one per architecture, see for instance
<c>tests/datawizard/locality.c</c>
</li>
</ul>

For ::STARPU_HISTORY_BASED, ::STARPU_REGRESSION_BASED, and
::STARPU_NL_REGRESSION_BASED, the dimensions of task data (both input
and output) are used as an index by default. ::STARPU_HISTORY_BASED uses a CRC
hash of the dimensions as an index to distinguish histories, and
::STARPU_REGRESSION_BASED and ::STARPU_NL_REGRESSION_BASED use the total
size as an index for the regression.

The starpu_perfmodel::size_base and starpu_perfmodel::footprint fields however
permit the application to override that, when for instance some of the data
do not matter for task cost (e.g. mere reference table), or when using sparse
structures (in which case it is the number of non-zeros which matter), or when
there is some hidden parameter such as the number of iterations, or when the
application actually has a very good idea of the complexity of the algorithm,
and just not the speed of the processor, etc.  The example in the directory
<c>examples/pi</c> uses this to include the number of iterations in the base
size. starpu_perfmodel::size_base should be used when the variance of the actual
performance is known (i.e. bigger return value is longer execution
time), and thus particularly useful for ::STARPU_REGRESSION_BASED or
::STARPU_NL_REGRESSION_BASED. starpu_perfmodel::footprint can be used when the
variance of the actual performance is unknown (irregular performance behavior,
etc.), and thus only useful for ::STARPU_HISTORY_BASED.
starpu_task_data_footprint() can be used as a base and combined with other
parameters through starpu_hash_crc32c_be() for instance.

StarPU will automatically determine when the performance model is calibrated,
or rather, it will assume the performance model is calibrated until the
application submits a task for which the performance can not be predicted. For
::STARPU_HISTORY_BASED, StarPU will require 10 (STARPU_CALIBRATE_MINIMUM)
measurements for a given size before estimating that an average can be taken as
estimation for further executions with the same size. For
::STARPU_REGRESSION_BASED and ::STARPU_NL_REGRESSION_BASED, StarPU will require
10 (STARPU_CALIBRATE_MINIMUM) measurements, and that the minimum measured
data size is smaller than 90% of the maximum measured data size (i.e. the
measurement interval is large enough for a regression to have a meaning).
Calibration can also be forced by setting the \ref STARPU_CALIBRATE environment
variable to <c>1</c>, or even reset by setting it to <c>2</c>.

How to use schedulers which can benefit from such performance model is explained
in \ref TaskSchedulingPolicy.

The same can be done for task energy consumption estimation, by setting
the field starpu_codelet::energy_model the same way as the field
starpu_codelet::model. Note: for now, the application has to give to
the energy consumption performance model a name which is different from
the execution time performance model.

The application can request time estimations from the StarPU performance
models by filling a task structure as usual without actually submitting
it. The data handles can be created by calling any of the functions
<c>starpu_*_data_register</c> with a <c>NULL</c> pointer and <c>-1</c>
node and the desired data sizes, and need to be unregistered as usual.
The functions starpu_task_expected_length() and
starpu_task_expected_energy() can then be called to get an estimation
of the task cost on a given arch. starpu_task_footprint() can also be
used to get the footprint used for indexing history-based performance
models. starpu_task_destroy() needs to be called to destroy the dummy
task afterwards. See <c>tests/perfmodels/regression_based.c</c> for an example.

The application can also request an on-the-fly XML report of the performance
model, by calling starpu_perfmodel_dump_xml() to print the report to a
<c>FILE*</c>.

\section Performance Monitoring Counters

This section presents the StarPU performance monitoring framework. It summarizes the objectives of the framework. It then introduces the entities involved in the framework. It presents the API of the framework, as well as some implementation details. It exposes the typical sequence of operations to plug an external tool to monitor a performance counter of StarPU.

\subsection Objectives

The objectives of this framework are to let external tools interface with StarPU to collect various performance metrics at runtime, in a generic, safe, extensible way. For that, it enables such tools to discover the available performance metrics in a particular StarPU build as well as the type of each performance counter value. It lets these tools build sets of performance counters to monitor, and then register listener callbacks to collect the measurement samples of these sets of performance counters at runtime.

\subsection Entities

The performance monitoring framework is built on a series of concepts and items, organized in a consistent way. The corresponding C language objects should be considered opaque by external tools, and should only be manipulated through proper function calls and accessors.

\subsubsection Performance Counter

The performance counter entity is the fundamental object of the framework, representing one piece of performance metrics, such as for instance the total number of tasks submitted so far, that is exported by StarPU and can be collected through the framework at runtime. A performance counter has a type and belongs to a scope. A performance counter is designated by a unique name and unique ID integer.

\subsubsection Performance Counter Type

A performance counter has a type. A type is designated by a unique name and unique ID number. Currently supported types include:

\verbatim
Type Name	Type Definition
"int32"	32-bit signed integers
"int64"	64-bit signed integers
"float"	32-bit single-precision floating point
"double"	64-bit double-precision floating point
\endverbatim

\subsubsection Performance Counter Scope

A performance counter belongs to a scope. The scope of a counter defines the context considered for computing the corresponding performance counter. A scope is designated with a unique name and unique ID number. Currently defined scopes include:

\verbatim
Scope Name	Scope Definition
"global"	Counter is global to the StarPU instance
"per_worker"	Counter is within the scope of a thread worker
"per_codelet"	Counter is within the scope of a task codelet 
\endverbatim

\subsubsection Performance Counter Set

A performance counter set is a subset of the performance counters belonging to the same scope. Each counter of the scope can be in the enabled or disabled state in a performance counter set. A performance counter set enables a performance monitoring tool to indicate the set of counters to be collected for a particular listener callback.

\subsubsection Performance Counter Sample

A performance counter sample corresponds to one sample of collected measurement values of a performance counter set. Only the values corresponding to enabled counters in the sample's counter set should be observed by the listener callback. Whether the sample contains valid values for counters disabled in the set is unspecified.

\subsubsection Performance Counter Listener

A performance counter listener is a callback function registered by some external tool to monitor a set of performance counters in a particular scope. It is called each time a new performance counter sample is ready to be observed. The sample object should not be accessed outside of the callback.

\subsubsection Application Programming Interface

The API of the performance monitoring framework is defined in the "starpu_perf_monitoring.h" public header file of StarPU. This header file is automatically included with "starpu.h". An example of use of the routines is given in Section 3.6.

\subsubsection Scope Related Routines

\verbatim
Function Name	Function Definition
starpu_perf_counter_scope_name_to_id	Translate scope name constant string to scope id
starpu_perf_counter_scope_id_to_name	Translate scope id to scope name constant string
\endverbatim

\subsubsection Type Related Routines

\verbatim
Function Name	Function Definition
starpu_perf_counter_type_name_to_id	Translate type name constant string to type id
starpu_perf_counter_type_id_to_name	Translate type id to type name constant string
\endverbatim


\subsubsection Counter Related Routines

\verbatim
Function Name	Function Definition
starpu_perf_counter_nb	Return the number of performance counters for the given scope
starpu_perf_counter_name_to_id	Translate a performance counter name to its id
starpu_perf_counter_nth_to_id	Translate a performance counter rank in its scope to its counter id
starpu_perf_counter_id_to_name	Translate a counter id to its name constant string
starpu_perf_counter_get_type_id	Return the counter's type id
starpu_perf_counter_get_help_string	Return the counter's help string
starpu_perf_counter_list_avail	Display the list of counters defined in the given scope
starpu_perf_counter_list_all_avail	Display the list of counters defined in all scopes
\endverbatim


\subsubsection Counter Set Related Routines

\verbatim
Function Name	Function Definition
starpu_perf_counter_set_alloc	Allocate a new performance counter set
starpu_perf_counter_set_free	Free a performance counter set
starpu_perf_counter_set_enable_id	Enable a given counter in the set
starpu_perf_counter_set_disable_id	Disable a given counter in the set
\endverbatim


\subsubsection Listener Related Routines

\verbatim
Function Name	Function Definition
starpu_perf_counter_listener_init	Initialize a new performance counter listener
starpu_perf_counter_listener_exit	End a performance counter listener
starpu_perf_counter_set_global_listener	Set a listener for the global scope
starpu_perf_counter_set_per_worker_listener	Set a listener for the per_worker scope on a given worker
starpu_perf_counter_set_all_per_worker_listeners	Set a common listener for all workers
starpu_perf_counter_set_per_codelet_listener	Set a per_codelet listener for a codelet
starpu_perf_counter_unset_global_listener	Unset the global listener
starpu_perf_counter_unset_per_worker_listener	Unset the per_worker listener
starpu_perf_counter_unset_all_per_worker_listeners	Unset all per_worker listeners
starpu_perf_counter_unset_per_codelet_listener	Unset a per_codelet listener
\endverbatim


\subsubsection Sample Related Routines

\verbatim
Function Name	Function Definition
starpu_perf_counter_sample_get_int32_value	Read an int32 counter value from a sample
starpu_perf_counter_sample_get_int64_value	Read an int64 counter value from a sample
starpu_perf_counter_sample_get_float_value	Read a float counter value from a sample
starpu_perf_counter_sample_get_double_value	Read a double counter value from a sample
\endverbatim

\subsection Implementation Details

\subsubsection Performance Counter Registration

Each module of StarPU can export performance counters. In order to do so, modules that need to export some counters define a registration function that is called at StarPU initialization time. This function is responsible for calling the "_starpu_perf_counter_register()" function once for each counter it exports, to let the framework know about the list of counters managed by the module. It also registers performance sample updater callbacks for the module, one for each scope for which it exports counters.

\subsubsection Performance Sample Updaters

The updater callback for a module and scope combination is internally called every time a sample for a set of performance counter must be updated. Thus, the updated callback is responsible for filling the sample's selected counters with the counter values found at the time of the call.
Global updaters are currently called at task submission time, as well as any blocking tasks management function of the StarPU API, such as the "starpu_task_wait_for_all()", which waits for the completion of all tasks submitted up to this point.
Per-worker updaters are currently called at the level of StarPU's drivers, that is, the modules in charge of task execution of hardware-specific worker threads. The actual calls occur in-between the execution of tasks.
Per-codelet updaters are currently called both at task submission time, and at the level of StarPU's drivers together with the per-worker updaters.

A performance sample object is locked during the sample collection. The locking prevents the following issues:

<ul>
<li>The listener of sample being changed during sample collection;
<li>The set of counters enabled for a sample being changed;
<li>Conflicting concurrent updates;
<li>Updates while the sample is being read by the listener.
</ul>

The location of the updaters' calls is chosen to minimize the sequentialization effect of the locking, in order to limit the level of interference of the monitoring process. For Global updaters, the calls are performed only on the application thread(s) in charge of submitting tasks. Since, in most cases, only a single application thread submits tasks, the sequentialization effect is moderate. Per-worker updates are local to their worker, thus here again the sample lock is un-contented, unless the external monitoring tool frequently changes the set of enabled counters in the sample.

\subsubsection Counter operations

In practice the sample updaters only take snapshots of the actual performance counters. The performance counters themselves are updated with ad-hoc procedures depending on each counter. Such procedures typically involve atomic operations. While operations such as atomic increments or decrements on integer values are readily available, this is not the case for more complex operations such as min/max for computing peak value counters (for instance in the global and per-codelet counters for peak number of submitted tasks and peak number of ready tasks waiting for execution), and this is also not the case for computations on floating point data (used for instance in computing cumulated execution time of tasks, either per worker or per codelet). The performance monitoring framework therefore supplies such missing routines, for the internal use of StarPU.

\subsubsection Runtime checks

The performance monitoring framework features a comprehensive set of runtime checks to verify that both StarPU and some external tool do not access a performance counter with the wrong typed routines, to quickly detect situations of mismatch that can result from the evolution of multiple pieces of software at distinct paces. Moreover, no StarPU data structure is accessed directly either by the external code making use of the performance monitoring framework. The use of the C enum constants is optional; referring to values through constant strings is available when more robustness is desired. These runtime checks enable the framework to be extensible. Moreover, while the framework's counters currently are permanently compiled in, they could be made optional at compile time, for instance to suppress any overhead once the analysis and optimization process has been completed by the programmer. Thanks to the runtime discovery of available counters, the applicative code, or an intermediate layer such as skeleton layer acting on its behalf, would then be able to adapt to performance analysis builds versus optimized builds.

\subsection Exported Counters

\subsubsection Global Scope

\verbatim
Counter Name	Counter Definition
starpu.task.g_total_submitted	Total number of tasks submitted
starpu.task.g_peak_submitted	Maximum number of tasks submitted, waiting for dependencies resolution at any time
starpu.task.g_peak_ready	Maximum number of tasks ready for execution, waiting for an execution slot at any time
\endverbatim


\subsubsection Per-worker Scope

\verbatim
Counter Name	Counter Definition
starpu.task.w_total_executed	Total number of tasks executed on a given worker
starpu.task.w_cumul_execution_time	Cumulated execution time of tasks executed on a given worker
\endverbatim


\subsubsection Per-Codelet Scope

\verbatim
Counter Name	Counter Definition
starpu.task.c_total_submitted	Total number of submitted tasks for a given codelet
starpu.task.c_peak_submitted     	Maximum number of submitted tasks for a given codelet waiting for dependencies resolution at any time
starpu.task.c_peak_ready      	Maximum number of ready tasks for a given codelet waiting for an execution slot at any time
starpu.task.c_total_executed      	Total number of executed tasks for a given codelet
starpu.task.c_cumul_execution_time	Cumulated execution time of tasks for a given codelet
\endverbatim

\subsection Sequence of operations

This section presents a typical sequence of operations to interface an external tool with some StarPU performance counters. In this example, the counters monitored are the per-worker total number of executed tasks ("starpu.task.w_total_executed") and the tasks' cumulated execution time ("starpu.task.w_cumul_execution_time").

<b>Step 0: Initialize StarPU</b>

StarPU must first be initialized, by a call to starpu_init(), for performance counters to become available, since each module of StarPU registers the performance counters it exports during that initialization phase.

\verbatim
int ret = starpu_init(NULL);	
\endverbatim


<b>Step 1: Allocate a counter set</b>

A counter set has to be allocated on the per-worker scope. The per-worker scope id can be obtained by name, or with the pre-defined enum value starpu_perf_counter_scope_per_worker.

\verbatim
enum starpu_perf_counter_scope w_scope = starpu_perf_counter_scope_per_worker;
struct starpu_perf_counter_set *w_set = starpu_perf_counter_set_alloc(w_scope);	
\endverbatim


<b>Step 2: Get the counter IDs</b>
Each performance counter has a unique ID used to refer to it in subsequent calls to the performance monitoring framework.

\verbatim
int id_w_total_executed = starpu_perf_counter_name_to_id(w_scope, 
                                                     "starpu.task.w_total_executed");

int id_w_cumul_execution_time = starpu_perf_counter_name_to_id(w_scope, 
                                               "starpu.task.w_cumul_execution_time");
\endverbatim


<b>Step 3: Enable the counters in the counter set</b>

This step indicates which counters will be collected into performance monitoring samples for the listeners referring to this counter set.

\verbatim
starpu_perf_counter_set_enable_id(w_set, id_w_total_executed);
starpu_perf_counter_set_enable_id(w_set, id_w_cumul_execution_time);	
\endverbatim


<b>Step 4: Write a listener callback</b>

This callback will be triggered when a sample becomes available. Upon execution, it reads the values for the two counters from the sample and displays these values, for the sake of the example.

\verbatim
void w_listener_cb(struct starpu_perf_counter_listener *listener, 
                   struct starpu_perf_counter_sample   *sample, 
                   void *context)
{
  int32_t w_total_executed = 
             starpu_perf_counter_sample_get_int32_value(sample, id_w_total_executed);
  
  double w_cumul_execution_time = 
      starpu_perf_counter_sample_get_double_value(sample, id_w_cumul_execution_time);
 
  printf("worker[%d]: w_total_executed = %d, w_cumul_execution_time = %lf\n", 
                                 starpu_worker_get_id(), 
                                 w_total_executed, 
                                 w_cumul_execution_time);
}	
\endverbatim


<b>Step 5: Initialize the listener</b>

This step allocates the listener structure and prepares it to listen to the selected set of per-worker counters. However, it is not actually active until Step 6, once it is attached to one or more worker.

\verbatim
struct starpu_perf_counter_listener * w_listener =   
                       starpu_perf_counter_listener_init(w_set, w_listener_cb, NULL);
\endverbatim


<b>Step 6: Set the listener on all workers</b>
This step actually makes the listener active, in this case on every StarPU worker thread. 

\verbatim
starpu_perf_counter_set_all_per_worker_listeners(w_listener);
\endverbatim


After this step, any task assigned to a worker will be counted in that worker selected performance counters, and reported to the listener.



\section Performance Steering Knobs

This section presents the StarPU performance steering framework. It summarizes the objectives of the framework. It introduces the entities involved in the framework, and then details the API, implementation and sequence of operations.

\subsection Objectives

The objectives of this framework are to let external tools interface with StarPU, observe, and act at runtime on actionable performance steering knobs exported by StarPU, in a generic, safe, extensible way. It defines an API to let such external tools discover the available performance steering knobs in a particular StarPU revision of build, as well as the type of each knob.

\subsection Entities

\subsubsection Performance Steering Knob

The performance steering knob entity designates one runtime-actionable knob exported by StarPU. It may represent some setting, or some constant used within StarPU for a given purpose. The value of the knob is typed, it can be obtained or modified with the appropriate getter/setter routine. The knob belongs to a scope. A performance steering knob is designated with a unique name and unique ID number.

\subsubsection Knob Type

A performance steering knob has a type. A type is designated by a unique name and unique ID number. Currently supported types include:

\verbatim
Type Name	Type Definition
"int32"	32-bit signed integers
"int64"	64-bit signed integers
"float"	32-bit single precision floating point
"double"	64-bit double precision floating point
\endverbatim

On/Off knobs are defined as "int32" type, with value 0 for Off and value !0 for On, unless otherwise specified.

\subsubsection Knob Scope

A performance steering knob belongs to a scope. The scope of a knob defines the context considered for computing the corresponding knob. A scope is designated with a unique name and unique ID number. Currently defined scopes include:

\verbatim
Scope Name	Scope Definition
"global"	Knob is global to the StarPU instance
"per_worker"	Knob is within the scope of a thread worker
"per_scheduler"	Knob is within the scope of a scheduling policy instance
\endverbatim


\subsubsection Knob Group

The notion of Performance Steering Knob Group is currently internal to StarPU. It defines a series of knobs that are handled by the same couple of setter/getter functions internally. A knob group belongs to a knob scope.

\subsection Application Programming Interface

\subsubsection Scope Related Routines

\verbatim
Function Name	Function Definition
starpu_perf_knob_scope_name_to_id	Translate scope name constant string to scope id
starpu_perf_knob_scope_id_to_name	Translate scope id to scope name constant string
\endverbatim


\subsubsection Type Related Routines

\verbatim
Function Name	Function Definition
starpu_perf_knob_type_name_to_id	Translate type name constant string to type id
starpu_perf_knob_type_id_to_name	Translate type id to type name constant string
\endverbatim


\subsubsection Performance Steering Knob Related Routines

\verbatim
Function Name	Function Definition
starpu_perf_knob_nb	Return the number of performance steering knobs for the given scope
starpu_perf_knob_name_to_id	Translate a performance knob name to its id
starpu_perf_knob_nth_to_id	Translate a performance knob rank in its scope to its knob id
starpu_perf_knob_id_to_name	Translate a knob id to its name constant string
starpu_perf_knob_get_type_id	Return the knob's type id
starpu_perf_knob_get_help_string	Return the knob's help string
starpu_perf_knob_list_avail	Display the list of knobs defined in the given scope
starpu_perf_knob_list_all_avail	Display the list of knobs defined in all scopes
starpu_perf_knob_get_<SCOPE>_<TYPE>_value	Get knob value for given scope and type
starpu_perf_knob_set_<SCOPE>_<TYPE>_value	Set knob value for given scope and type
\endverbatim

\subsection Implementation Details

While the APIs of the monitoring and the steering frameworks share a similar design philosophy, the internals are significantly different. Since the effect of the steering knobs varies widely, there is no global locking scheme in place shared for all knobs. Instead, each knob gets its own procedures to get the value of a setting, or change it. To prevent code duplication, some related knobs may share getter/setter routines as knob groups.

The steering framework does not involve callback routines. Knob get operations proceed immediately, except for the possible delay in getting access to the knob value. Knob set operations also proceed immediately, not counting the exclusive access time, though their action result may be observed with some latency, depending on the knob and on the current workload. For instance, acting on a per-worker "starpu.worker.w_enable_worker_knob" to disable a worker thread may be observed only after the corresponding worker's assigned task queue becomes empty, since its actual effect is to prevent additional tasks to be queued to the worker, and not to migrate already queued tasks to another worker. Such design choices aim at providing a compromise between offering some steering capabilities and keeping the cost of supporting such steering capabilities to an acceptable level.

The framework is designed to be easily extensible. At StarPU initialization time, the framework calls initialization functions if StarPU modules to initialize the set of knobs they export. Knob get/set accessors can be shared among multiple knobs in a knob group. Thus, exporting a new knob is basically a matter of declaring it at initialization time, by specifying its name and value type, and either add its handling to an existing getter/setter pair of accessors in a knob group, or create a new group. As the performance monitoring framework, the performance steering framework is currently permanently enabled, but could be made optional at compile-time to separate testing builds from production builds.

\subsection Exported Steering Knobs

\subsubsection Global Scope

\verbatim
Knob Name	Knob Definition
starpu.global.g_calibrate_knob
	Enable/disable the calibration of performance models
starpu.global.g_enable_catch_signal_knob	Enable/disable the catching of UNIX signals
\endverbatim


\subsubsection Per-worker Scope

\verbatim
Knob Name	Knob Definition
starpu.worker.w_bind_to_pu_knob	Change the processing unit to which a worker thread is bound
starpu.worker.w_enable_worker_knob	Disable/re-enable a worker thread to be selected
for task execution
\endverbatim


\subsubsection Per-Scheduler Scope

\verbatim
Knob Name	Knob Definition
starpu.task.s_max_priority_cap_knob 	Set a capping maximum priority value for subsequently submitted tasks
starpu.task.s_min_priority_cap_knob	Set a capping minimum priority value for subsequently submitted tasks
starpu.dmda.s_alpha_knob 	Scaling factor for the Alpha constant for Deque Model schedulers to alter the weight of the estimated task execution time
starpu.dmda.s_beta_knob 	Scaling factor for the Beta constant for Deque Model schedulers to alter the weight of the estimated data transfer time for the task's input(s)
starpu.dmda.s_gamma_knob	Scaling factor for the Gamma constant for Deque Model schedulers to alter the weight of the estimated power consumption of the task
starpu.dmda.s_idle_power_knob	Scaling factor for the baseline Idle power consumption estimation of the corresponding processing unit
\endverbatim


\subsection Sequence of operations

This section presents an example of sequence of operations representing a typical use of the performance steering knobs exported by StarPU. In this example, a worker thread is temporarily barred from executing tasks. For that, the corresponding "starpu.worker.w_enable_worker_knob" of the worker, initially set to 1 (= enabled) is changed to 0 (= disabled).

<b>Step 0: Initialize StarPU</b>

StarPU must first be initialized, by a call to starpu_init(). Performance steering knobs only become available after this step, since each module of StarPU registers the knobs it exports during that initialization phase.

\verbatim
int ret = starpu_init(NULL);
\endverbatim


<b>Step 1: Get the knob ID</b>

Each performance steering knob has a unique ID used to refer to it in subsequent calls to the performance steering framework. The knob belongs to the "per_worker" scope.

\verbatim
int w_scope = starpu_perf_knob_scope_name_to_id("per_worker");
int w_enable_id = starpu_perf_knob_name_to_id(w_scope,
                                               "starpu.worker.w_enable_worker_knob");
\endverbatim


<b>Step 2: Get the knob current value</b>

This knob is an On/Off knob. Its value type is therefore a 32-bit integer, with value 0 for Off and value !0 for On. The getter functions for per-worker knobs expect the knob ID as first argument, and the worker ID as second argument. Here the getter call obtains the value of worker 5.

\verbatim
int32_t val = starpu_perf_knob_get_per_worker_int32_value(w_enable_id, 5);
\endverbatim


<b>Step 3: Set the knob current value</b>

The setter functions for per-worker knobs expect the knob ID as first argument, the worker ID as second argument, and the new value as third argument. Here, the value for worker 5 is set to 0 to temporarily bar the worker thread from accepting new tasks for execution.

\verbatim
starpu_perf_knob_set_per_worker_int32_value(w_enable_id, 5, 0);
\endverbatim


Subsequently setting the value of the knob back to 1 enables the corresponding to accept new tasks for execution again.

\verbatim
starpu_perf_knob_set_per_worker_int32_value(w_enable_id, 5, 1);
\endverbatim





*/