StarPU Handbook
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Offline Performance Tools

To get an idea of what is happening, a lot of performance feedback is available, detailed in this chapter. The various informations should be checked for.

  • What does the Gantt diagram look like? (see Creating a Gantt Diagram)
    • If it's mostly green (tasks running in the initial context) or context specific color prevailing, then the machine is properly utilized, and perhaps the codelets are just slow. Check their performance, see Performance Of Codelets.
    • If it's mostly purple (FetchingInput), tasks keep waiting for data transfers, do you perhaps have far more communication than computation? Did you properly use CUDA streams to make sure communication can be overlapped? Did you use data-locality aware schedulers to avoid transfers as much as possible?
    • If it's mostly red (Blocked), tasks keep waiting for dependencies, do you have enough parallelism? It might be a good idea to check what the DAG looks like (see Creating a DAG With Graphviz).
    • If only some workers are completely red (Blocked), for some reason the scheduler didn't assign tasks to them. Perhaps the performance model is bogus, check it (see Performance Of Codelets). Do all your codelets have a performance model? When some of them don't, the schedulers switches to a greedy algorithm which thus performs badly.

You can also use the Temanejo task debugger (see Using The Temanejo Task Debugger) to visualize the task graph more easily.

Off-line Performance Feedback

Generating Traces With FxT

StarPU can use the FxT library (see https://savannah.nongnu.org/projects/fkt/) to generate traces with a limited runtime overhead.

You can either get a tarball:

$ wget http://download.savannah.gnu.org/releases/fkt/fxt-0.2.11.tar.gz

or use the FxT library from CVS (autotools are required):

$ cvs -d :pserver:anonymous\@cvs.sv.gnu.org:/sources/fkt co FxT
$ ./bootstrap

Compiling and installing the FxT library in the $FXTDIR path is done following the standard procedure:

$ ./configure --prefix=$FXTDIR
$ make
$ make install

In order to have StarPU to generate traces, StarPU should be configured with the option --with-fxt :

$ ./configure --with-fxt=$FXTDIR

Or you can simply point the PKG_CONFIG_PATH to $FXTDIR/lib/pkgconfig and pass --with-fxt to ./configure

When FxT is enabled, a trace is generated when StarPU is terminated by calling starpu_shutdown(). The trace is a binary file whose name has the form prof_file_XXX_YYY where XXX is the user name, and YYY is the pid of the process that used StarPU. This file is saved in the /tmp/ directory by default, or by the directory specified by the environment variable STARPU_FXT_PREFIX.

The additional configure option --enable-fxt-lock can be used to generate trace events which describes the locks behaviour during the execution. It is however very heavy and should not be used unless debugging StarPU's internal locking.

The environment variable STARPU_FXT_TRACE can be set to 0 to disable the generation of the prof_file_XXX_YYY file.

Creating a Gantt Diagram

When the FxT trace file prof_file_something has been generated, it is possible to generate a trace in the Paje format by calling:

$ starpu_fxt_tool -i /tmp/prof_file_something

Or alternatively, setting the environment variable STARPU_GENERATE_TRACE to 1 before application execution will make StarPU do it automatically at application shutdown.

This will create a file paje.trace in the current directory that can be inspected with the ViTE (http://vite.gforge.inria.fr/) trace visualizing open-source tool. It is possible to open the file paje.trace with ViTE by using the following command:

$ vite paje.trace

To get names of tasks instead of "unknown", fill the optional starpu_codelet::name, or use a performance model for them.

In the MPI execution case, STARPU_GENERATE_TRACE will not work as expected (each node will try to generate paje.trace, thus mixing outputs...), you have to collect the trace files from the MPI nodes, and specify them all on the command starpu_fxt_tool, for instance:

$ starpu_fxt_tool -i /tmp/prof_file_something*

By default, all tasks are displayed using a green color. To display tasks with varying colors, pass option -c to starpu_fxt_tool.

By default, the trace contains all informations. To reduce the trace size, various -no-foo options can be passed to starpu_fxt_tool, see starpu_fxt_tool –help .

To identify tasks precisely, the application can set the starpu_task::tag_id field of the task (or use STARPU_TAG_ONLY when using starpu_task_insert()), and the value of the tag will show up in the trace.

It can also set the starpu_task::name field of the task (or use STARPU_NAME) when using starpu_task_insert()), to replace in traces the name of the codelet with an arbitrarily chosen name.

It can also set the iteration number, by just calling starpu_iteration_push() at the beginning of submission loops and starpu_iteration_pop() at the end of submission loops. These iteration numbers will show up in traces for all tasks submitted from there.

Coordinates can also be given to data with the starpu_data_set_coordinates() or starpu_data_set_coordinates_array() function. In the trace, tasks will then be assigned the coordinates of the first data they write to.

Traces can also be inspected by hand by using the tool fxt_print, for instance:

$ fxt_print -o -f /tmp/prof_file_something

Timings are in nanoseconds (while timings as seen in ViTE are in milliseconds).

Creating a DAG With Graphviz

When the FxT trace file prof_file_something has been generated, it is possible to generate a task graph in the DOT format by calling:

$ starpu_fxt_tool -i /tmp/prof_file_something

This will create a dag.dot file in the current directory. This file is a task graph described using the DOT language. It is possible to get a graphical output of the graph by using the graphviz library:

$ dot -Tpdf dag.dot -o output.pdf

Getting Task Details

When the FxT trace file prof_file_something has been generated, details on the executed tasks can be retrieved by calling:

$ starpu_fxt_tool -i /tmp/prof_file_something

This will create a tasks.rec file in the current directory. This file is in the recutils format, i.e. Field: value lines, and empty lines to separate each task. This can be used as a convenient input for various ad-hoc analysis tools. By default it only contains information about the actual execution. Performance models can be obtained by running starpu_tasks_rec_complete on it:

$ starpu_tasks_rec_complete tasks.rec tasks2.rec

which will add EstimatedTime lines which contain the performance model-estimated time (in ┬Ás) for each worker starting from 0. Since it needs the performance models, it needs to be run the same way as the application execution, or at least with STARPU_HOSTNAME set to the hostname of the machine used for execution, to get the performance models of that machine.

Monitoring Activity

When the FxT trace file prof_file_something has been generated, it is possible to generate an activity trace by calling:

$ starpu_fxt_tool -i /tmp/prof_file_something

This will create a file activity.data in the current directory. A profile of the application showing the activity of StarPU during the execution of the program can be generated:

$ starpu_workers_activity activity.data

This will create a file named activity.eps in the current directory. This picture is composed of two parts. The first part shows the activity of the different workers. The green sections indicate which proportion of the time was spent executed kernels on the processing unit. The red sections indicate the proportion of time spent in StartPU: an important overhead may indicate that the granularity may be too low, and that bigger tasks may be appropriate to use the processing unit more efficiently. The black sections indicate that the processing unit was blocked because there was no task to process: this may indicate a lack of parallelism which may be alleviated by creating more tasks when it is possible.

The second part of the picture activity.eps is a graph showing the evolution of the number of tasks available in the system during the execution. Ready tasks are shown in black, and tasks that are submitted but not schedulable yet are shown in grey.

Getting Modular Schedular Animation

When using modular schedulers (i.e. schedulers which use a modular architecture, and whose name start with "modular-"), the command

$ starpu_fxt_tool -i /tmp/prof_file_something

will also produce a trace.html file which can be viewed in a javascript-enabled web browser. It shows the flow of tasks between the components of the modular scheduler.

Limiting The Scope Of The Trace

For computing statistics, it is useful to limit the trace to a given portion of the time of the whole execution. This can be achieved by calling

before calling starpu_init(), to prevent tracing from starting immediately. Then

and

can be used around the portion of code to be traced. This will show up as marks in the trace, and states of workers will only show up for that portion.

Performance Of Codelets

The performance model of codelets (see Performance Model Example) can be examined by using the tool starpu_perfmodel_display:

$ starpu_perfmodel_display -l
file: <malloc_pinned.hannibal>
file: <starpu_slu_lu_model_21.hannibal>
file: <starpu_slu_lu_model_11.hannibal>
file: <starpu_slu_lu_model_22.hannibal>
file: <starpu_slu_lu_model_12.hannibal>

Here, the codelets of the example lu are available. We can examine the performance of the kernel 22 (in micro-seconds), which is history-based:

$ starpu_perfmodel_display -s starpu_slu_lu_model_22
performance model for cpu
# hash      size       mean          dev           n
57618ab0    19660800   2.851069e+05  1.829369e+04  109
performance model for cuda_0
# hash      size       mean          dev           n
57618ab0    19660800   1.164144e+04  1.556094e+01  315
performance model for cuda_1
# hash      size       mean          dev           n
57618ab0    19660800   1.164271e+04  1.330628e+01  360
performance model for cuda_2
# hash      size       mean          dev           n
57618ab0    19660800   1.166730e+04  3.390395e+02  456

We can see that for the given size, over a sample of a few hundreds of execution, the GPUs are about 20 times faster than the CPUs (numbers are in us). The standard deviation is extremely low for the GPUs, and less than 10% for CPUs.

This tool can also be used for regression-based performance models. It will then display the regression formula, and in the case of non-linear regression, the same performance log as for history-based performance models:

$ starpu_perfmodel_display -s non_linear_memset_regression_based
performance model for cpu_impl_0
	Regression : #sample = 1400
	Linear: y = alpha size ^ beta
		alpha = 1.335973e-03
		beta = 8.024020e-01
	Non-Linear: y = a size ^b + c
		a = 5.429195e-04
		b = 8.654899e-01
		c = 9.009313e-01
# hash		size		mean		stddev		n
a3d3725e	4096           	4.763200e+00   	7.650928e-01   	100
870a30aa	8192           	1.827970e+00   	2.037181e-01   	100
48e988e9	16384          	2.652800e+00   	1.876459e-01   	100
961e65d2	32768          	4.255530e+00   	3.518025e-01   	100
...

The same can also be achieved by using StarPU's library API, see Performance Model and notably the function starpu_perfmodel_load_symbol(). The source code of the tool starpu_perfmodel_display can be a useful example.

The tool starpu_perfmodel_plot can be used to draw performance models. It writes a .gp file in the current directory, to be run with the tool gnuplot, which shows the corresponding curve.

starpu_non_linear_memset_regression_based.png

When the field starpu_task::flops is set (or STARPU_FLOPS is passed to starpu_task_insert()), starpu_perfmodel_plot can directly draw a GFlops curve, by simply adding the -f option:

$ starpu_perfmodel_plot -f -s chol_model_11

This will however disable displaying the regression model, for which we can not compute GFlops.

starpu_chol_model_11_type.png

When the FxT trace file prof_file_something has been generated, it is possible to get a profiling of each codelet by calling:

$ starpu_fxt_tool -i /tmp/prof_file_something
$ starpu_codelet_profile distrib.data codelet_name

This will create profiling data files, and a distrib.data.gp file in the current directory, which draws the distribution of codelet time over the application execution, according to data input size.

distrib_data.png

This is also available in the tool starpu_perfmodel_plot, by passing it the fxt trace:

$ starpu_perfmodel_plot -s non_linear_memset_regression_based -i /tmp/prof_file_foo_0

It will produce a .gp file which contains both the performance model curves, and the profiling measurements.

starpu_non_linear_memset_regression_based_2.png

If you have the statistical tool R installed, you can additionally use

$ starpu_codelet_histo_profile distrib.data

Which will create one .pdf file per codelet and per input size, showing a histogram of the codelet execution time distribution.

distrib_data_histo.png

Trace Statistics

More than just codelet performance, it is interesting to get statistics over all kinds of StarPU states (allocations, data transfers, etc.). This is particularly useful to check what may have gone wrong in the accurracy of the simgrid simulation.

This requires the R statistical tool, with the plyr, ggplot2 and data.table packages. If your system distribution does not have packages for these, one can fetch them from CRAN:

$ R
> install.packages("plyr")
> install.packages("ggplot2")
> install.packages("data.table")
> install.packages("knitr")

The pj_dump tool from pajeng is also needed (see https://github.com/schnorr/pajeng)

One can then get textual or .csv statistics over the trace states:

$ starpu_paje_state_stats -v native.trace simgrid.trace
"Value"         "Events_native.csv" "Duration_native.csv" "Events_simgrid.csv" "Duration_simgrid.csv"
"Callback"      220                 0.075978              220                  0
"chol_model_11" 10                  565.176               10                   572.8695
"chol_model_21" 45                  9184.828              45                   9170.719
"chol_model_22" 165                 64712.07              165                  64299.203
$ starpu_paje_state_stats native.trace simgrid.trace

And one can plot histograms of execution times, of several states for instance:

$ starpu_paje_draw_histogram -n chol_model_11,chol_model_21,chol_model_22 native.trace simgrid.trace

and see the resulting pdf file:

paje_draw_histogram.png

A quick statistical report can be generated by using:

$ starpu_paje_summary native.trace simgrid.trace

it includes gantt charts, execution summaries, as well as state duration charts and time distribution histograms.

Other external Paje analysis tools can be used on these traces, one just needs to sort the traces by timestamp order (which not guaranteed to make recording more efficient):

$ starpu_paje_sort paje.trace

Theoretical Lower Bound On Execution Time

StarPU can record a trace of what tasks are needed to complete the application, and then, by using a linear system, provide a theoretical lower bound of the execution time (i.e. with an ideal scheduling).

The computed bound is not really correct when not taking into account dependencies, but for an application which have enough parallelism, it is very near to the bound computed with dependencies enabled (which takes a huge lot more time to compute), and thus provides a good-enough estimation of the ideal execution time.

Theoretical Lower Bound On Execution Time Example provides an example on how to use this.

Theoretical Lower Bound On Execution Time Example

For kernels with history-based performance models (and provided that they are completely calibrated), StarPU can very easily provide a theoretical lower bound for the execution time of a whole set of tasks. See for instance examples/lu/lu_example.c: before submitting tasks, call the function starpu_bound_start(), and after complete execution, call starpu_bound_stop(). starpu_bound_print_lp() or starpu_bound_print_mps() can then be used to output a Linear Programming problem corresponding to the schedule of your tasks. Run it through lp_solve or any other linear programming solver, and that will give you a lower bound for the total execution time of your tasks. If StarPU was compiled with the library glpk installed, starpu_bound_compute() can be used to solve it immediately and get the optimized minimum, in ms. Its parameter integer allows to decide whether integer resolution should be computed and returned

The deps parameter tells StarPU whether to take tasks, implicit data, and tag dependencies into account. Tags released in a callback or similar are not taken into account, only tags associated with a task are. It must be understood that the linear programming problem size is quadratic with the number of tasks and thus the time to solve it will be very long, it could be minutes for just a few dozen tasks. You should probably use lp_solve -timeout 1 test.pl -wmps test.mps to convert the problem to MPS format and then use a better solver, glpsol might be better than lp_solve for instance (the –pcost option may be useful), but sometimes doesn't manage to converge. cbc might look slower, but it is parallel. For lp_solve, be sure to try at least all the -B options. For instance, we often just use lp_solve -cc -B1 -Bb -Bg -Bp -Bf -Br -BG -Bd -Bs -BB -Bo -Bc -Bi , and the -gr option can also be quite useful. The resulting schedule can be observed by using the tool starpu_lp2paje, which converts it into the Paje format.

Data transfer time can only be taken into account when deps is set. Only data transfers inferred from implicit data dependencies between tasks are taken into account. Other data transfers are assumed to be completely overlapped.

Setting deps to 0 will only take into account the actual computations on processing units. It however still properly takes into account the varying performances of kernels and processing units, which is quite more accurate than just comparing StarPU performances with the fastest of the kernels being used.

The prio parameter tells StarPU whether to simulate taking into account the priorities as the StarPU scheduler would, i.e. schedule prioritized tasks before less prioritized tasks, to check to which extend this results to a less optimal solution. This increases even more computation time.

Memory Feedback

It is possible to enable memory statistics. To do so, you need to pass the option --enable-memory-stats when running configure. It is then possible to call the function starpu_data_display_memory_stats() to display statistics about the current data handles registered within StarPU.

Moreover, statistics will be displayed at the end of the execution on data handles which have not been cleared out. This can be disabled by setting the environment variable STARPU_MEMORY_STATS to 0.

For example, if you do not unregister data at the end of the complex example, you will get something similar to:

$ STARPU_MEMORY_STATS=0 ./examples/interface/complex
Complex[0] = 45.00 + 12.00 i
Complex[0] = 78.00 + 78.00 i
Complex[0] = 45.00 + 12.00 i
Complex[0] = 45.00 + 12.00 i
$ STARPU_MEMORY_STATS=1 ./examples/interface/complex
Complex[0] = 45.00 + 12.00 i
Complex[0] = 78.00 + 78.00 i
Complex[0] = 45.00 + 12.00 i
Complex[0] = 45.00 + 12.00 i

#---------------------
Memory stats:
#-------
Data on Node #3
#-----
Data : 0x553ff40
Size : 16

#--
Data access stats
/!\ Work Underway
Node #0
	Direct access : 4
	Loaded (Owner) : 0
	Loaded (Shared) : 0
	Invalidated (was Owner) : 0

Node #3
	Direct access : 0
	Loaded (Owner) : 0
	Loaded (Shared) : 1
	Invalidated (was Owner) : 0

#-----
Data : 0x5544710
Size : 16

#--
Data access stats
/!\ Work Underway
Node #0
	Direct access : 2
	Loaded (Owner) : 0
	Loaded (Shared) : 1
	Invalidated (was Owner) : 1

Node #3
	Direct access : 0
	Loaded (Owner) : 1
	Loaded (Shared) : 0
	Invalidated (was Owner) : 0

Data Statistics

Different data statistics can be displayed at the end of the execution of the application. To enable them, you need to pass the option --enable-stats when calling configure. When calling starpu_shutdown() various statistics will be displayed, execution, MSI cache statistics, allocation cache statistics, and data transfer statistics. The display can be disabled by setting the environment variable STARPU_STATS to 0.

$ ./examples/cholesky/cholesky_tag
Computation took (in ms)
518.16
Synthetic GFlops : 44.21
#---------------------
MSI cache stats :
TOTAL MSI stats	hit 1622 (66.23 %)	miss 827 (33.77 %)
...
$ STARPU_STATS=0 ./examples/cholesky/cholesky_tag
Computation took (in ms)
518.16
Synthetic GFlops : 44.21