mirror of https://github.com/python/cpython
208 lines
9.4 KiB
ReStructuredText
208 lines
9.4 KiB
ReStructuredText
.. highlight:: shell-session
|
|
|
|
.. _perf_profiling:
|
|
|
|
==============================================
|
|
Python support for the Linux ``perf`` profiler
|
|
==============================================
|
|
|
|
:author: Pablo Galindo
|
|
|
|
`The Linux perf profiler <https://perf.wiki.kernel.org>`_
|
|
is a very powerful tool that allows you to profile and obtain
|
|
information about the performance of your application.
|
|
``perf`` also has a very vibrant ecosystem of tools
|
|
that aid with the analysis of the data that it produces.
|
|
|
|
The main problem with using the ``perf`` profiler with Python applications is that
|
|
``perf`` only gets information about native symbols, that is, the names of
|
|
functions and procedures written in C. This means that the names and file names
|
|
of Python functions in your code will not appear in the output of ``perf``.
|
|
|
|
Since Python 3.12, the interpreter can run in a special mode that allows Python
|
|
functions to appear in the output of the ``perf`` profiler. When this mode is
|
|
enabled, the interpreter will interpose a small piece of code compiled on the
|
|
fly before the execution of every Python function and it will teach ``perf`` the
|
|
relationship between this piece of code and the associated Python function using
|
|
:doc:`perf map files <../c-api/perfmaps>`.
|
|
|
|
.. note::
|
|
|
|
Support for the ``perf`` profiler is currently only available for Linux on
|
|
select architectures. Check the output of the ``configure`` build step or
|
|
check the output of ``python -m sysconfig | grep HAVE_PERF_TRAMPOLINE``
|
|
to see if your system is supported.
|
|
|
|
For example, consider the following script:
|
|
|
|
.. code-block:: python
|
|
|
|
def foo(n):
|
|
result = 0
|
|
for _ in range(n):
|
|
result += 1
|
|
return result
|
|
|
|
def bar(n):
|
|
foo(n)
|
|
|
|
def baz(n):
|
|
bar(n)
|
|
|
|
if __name__ == "__main__":
|
|
baz(1000000)
|
|
|
|
We can run ``perf`` to sample CPU stack traces at 9999 hertz::
|
|
|
|
$ perf record -F 9999 -g -o perf.data python my_script.py
|
|
|
|
Then we can use ``perf report`` to analyze the data:
|
|
|
|
.. code-block:: shell-session
|
|
|
|
$ perf report --stdio -n -g
|
|
|
|
# Children Self Samples Command Shared Object Symbol
|
|
# ........ ........ ............ .......... .................. ..........................................
|
|
#
|
|
91.08% 0.00% 0 python.exe python.exe [.] _start
|
|
|
|
|
---_start
|
|
|
|
|
--90.71%--__libc_start_main
|
|
Py_BytesMain
|
|
|
|
|
|--56.88%--pymain_run_python.constprop.0
|
|
| |
|
|
| |--56.13%--_PyRun_AnyFileObject
|
|
| | _PyRun_SimpleFileObject
|
|
| | |
|
|
| | |--55.02%--run_mod
|
|
| | | |
|
|
| | | --54.65%--PyEval_EvalCode
|
|
| | | _PyEval_EvalFrameDefault
|
|
| | | PyObject_Vectorcall
|
|
| | | _PyEval_Vector
|
|
| | | _PyEval_EvalFrameDefault
|
|
| | | PyObject_Vectorcall
|
|
| | | _PyEval_Vector
|
|
| | | _PyEval_EvalFrameDefault
|
|
| | | PyObject_Vectorcall
|
|
| | | _PyEval_Vector
|
|
| | | |
|
|
| | | |--51.67%--_PyEval_EvalFrameDefault
|
|
| | | | |
|
|
| | | | |--11.52%--_PyLong_Add
|
|
| | | | | |
|
|
| | | | | |--2.97%--_PyObject_Malloc
|
|
...
|
|
|
|
As you can see, the Python functions are not shown in the output, only ``_PyEval_EvalFrameDefault``
|
|
(the function that evaluates the Python bytecode) shows up. Unfortunately that's not very useful because all Python
|
|
functions use the same C function to evaluate bytecode so we cannot know which Python function corresponds to which
|
|
bytecode-evaluating function.
|
|
|
|
Instead, if we run the same experiment with ``perf`` support enabled we get:
|
|
|
|
.. code-block:: shell-session
|
|
|
|
$ perf report --stdio -n -g
|
|
|
|
# Children Self Samples Command Shared Object Symbol
|
|
# ........ ........ ............ .......... .................. .....................................................................
|
|
#
|
|
90.58% 0.36% 1 python.exe python.exe [.] _start
|
|
|
|
|
---_start
|
|
|
|
|
--89.86%--__libc_start_main
|
|
Py_BytesMain
|
|
|
|
|
|--55.43%--pymain_run_python.constprop.0
|
|
| |
|
|
| |--54.71%--_PyRun_AnyFileObject
|
|
| | _PyRun_SimpleFileObject
|
|
| | |
|
|
| | |--53.62%--run_mod
|
|
| | | |
|
|
| | | --53.26%--PyEval_EvalCode
|
|
| | | py::<module>:/src/script.py
|
|
| | | _PyEval_EvalFrameDefault
|
|
| | | PyObject_Vectorcall
|
|
| | | _PyEval_Vector
|
|
| | | py::baz:/src/script.py
|
|
| | | _PyEval_EvalFrameDefault
|
|
| | | PyObject_Vectorcall
|
|
| | | _PyEval_Vector
|
|
| | | py::bar:/src/script.py
|
|
| | | _PyEval_EvalFrameDefault
|
|
| | | PyObject_Vectorcall
|
|
| | | _PyEval_Vector
|
|
| | | py::foo:/src/script.py
|
|
| | | |
|
|
| | | |--51.81%--_PyEval_EvalFrameDefault
|
|
| | | | |
|
|
| | | | |--13.77%--_PyLong_Add
|
|
| | | | | |
|
|
| | | | | |--3.26%--_PyObject_Malloc
|
|
|
|
|
|
|
|
How to enable ``perf`` profiling support
|
|
----------------------------------------
|
|
|
|
``perf`` profiling support can be enabled either from the start using
|
|
the environment variable :envvar:`PYTHONPERFSUPPORT` or the
|
|
:option:`-X perf <-X>` option,
|
|
or dynamically using :func:`sys.activate_stack_trampoline` and
|
|
:func:`sys.deactivate_stack_trampoline`.
|
|
|
|
The :mod:`!sys` functions take precedence over the :option:`!-X` option,
|
|
the :option:`!-X` option takes precedence over the environment variable.
|
|
|
|
Example, using the environment variable::
|
|
|
|
$ PYTHONPERFSUPPORT=1 python script.py
|
|
$ perf report -g -i perf.data
|
|
|
|
Example, using the :option:`!-X` option::
|
|
|
|
$ python -X perf script.py
|
|
$ perf report -g -i perf.data
|
|
|
|
Example, using the :mod:`sys` APIs in file :file:`example.py`:
|
|
|
|
.. code-block:: python
|
|
|
|
import sys
|
|
|
|
sys.activate_stack_trampoline("perf")
|
|
do_profiled_stuff()
|
|
sys.deactivate_stack_trampoline()
|
|
|
|
non_profiled_stuff()
|
|
|
|
...then::
|
|
|
|
$ python ./example.py
|
|
$ perf report -g -i perf.data
|
|
|
|
|
|
How to obtain the best results
|
|
------------------------------
|
|
|
|
For best results, Python should be compiled with
|
|
``CFLAGS="-fno-omit-frame-pointer -mno-omit-leaf-frame-pointer"`` as this allows
|
|
profilers to unwind using only the frame pointer and not on DWARF debug
|
|
information. This is because as the code that is interposed to allow ``perf``
|
|
support is dynamically generated it doesn't have any DWARF debugging information
|
|
available.
|
|
|
|
You can check if your system has been compiled with this flag by running::
|
|
|
|
$ python -m sysconfig | grep 'no-omit-frame-pointer'
|
|
|
|
If you don't see any output it means that your interpreter has not been compiled with
|
|
frame pointers and therefore it may not be able to show Python functions in the output
|
|
of ``perf``.
|