Issue 36139: release GIL on mmap dealloc

munmap() can take a long time. I think mmap_object_dealloc can trivially release the GIL around this operation. Something similar was already mentioned in https://bugs.python.org/issue1572968 but a general patch was never provided. The dealloc case alone is significant enough to deserve fixing.

Your patch is modifying obmalloc.
It uses MAP_PRIVATE|MAP_ANONYMOUS mmap.
Does it really take log time?  How?

#1572968 is different.  It is mmap module.  It can use file-backed mmap.  It is I/O.

Sorry, I messed up.  Your PR is changing mmap module, not obmalloc.

Yes, this is mmap module. I found this to be slow for posix-shm-backed mmaps. Several milliseconds, like 20ms for a 128 MB object. Maybe the same can happen with private|anon mmaps? I will follow up with more numbers.

munmap() of private maps is usually pretty fast but not negligible (2 ms for 1GB). Shared maps are much slower. For some reason, shared maps explicitly backed by POSIX shared memory stand in between but are still pretty slow.

If someone cares about file-backed mmaps I can test those too. I thought this is already significant to justify releasing the GIL.


This is on Linux 4.4:


shared anon mmap 1048576 bytes
mmap time             6,393 ns
write time          449,062 ns
munmap time         100,205 ns
--
private anon mmap 1048576 bytes
mmap time             2,168 ns
write time          308,966 ns
munmap time          36,930 ns
--
posix shm + mmap 1048576 bytes
mmap time            13,299 ns
write time          369,305 ns
close time            1,545 ns
munmap time          26,759 ns
--

shared anon mmap 134217728 bytes
mmap time             4,641 ns
write time       64,508,536 ns
munmap time      13,592,556 ns
--
private anon mmap 134217728 bytes
mmap time             6,116 ns
write time       25,402,084 ns
munmap time         388,976 ns
--
posix shm + mmap 134217728 bytes
mmap time            29,034 ns
write time       66,826,645 ns
close time            3,707 ns
munmap time       3,475,977 ns
--

shared anon mmap 1073741824 bytes
mmap time            11,127 ns
write time      508,227,373 ns
munmap time      94,885,306 ns
--
private anon mmap 1073741824 bytes
mmap time             7,133 ns
write time      199,933,903 ns
munmap time       2,361,036 ns
--
posix shm + mmap 1073741824 bytes
mmap time            24,868 ns
write time      527,566,819 ns
close time            4,015 ns
munmap time      21,179,674 ns
--

The change sounds like a good idea and should be backported, too.

IIRC mmap() performance also depends on MMU and TLB speed. In the past I have seen paravirtualized systems with poor MMU performance that caused fork() to be slow and Redis to hang.

New changeset bb9593af0ac835b93c2834d44b72fa34e30efed0 by Benjamin Peterson (Davide Rizzo) in branch 'master':
closes bpo-36139: release GIL around munmap(). (GH-12073)
https://github.com/python/cpython/commit/bb9593af0ac835b93c2834d44b72fa34e30efed0

This change broke the Windows 7 buildbot:
https://buildbot.python.org/all/#/builders/40/builds/1745

Example:

0:01:34 [ 66/420/1] test_mmap crashed (Exit code 2147483651) -- running: test_tools (1 min 22 sec)
Fatal Python error: Python memory allocator called without holding the GIL

Current thread 0x00000de4 (most recent call first):
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\test_mmap.py", line 645 in test_crasher_on_windows
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\case.py", line 642 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\case.py", line 702 in __call__
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 122 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 84 in __call__
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 122 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 84 in __call__
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 122 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 84 in __call__
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\runner.py", line 176 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\support\__init__.py", line 1968 in _run_suite
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\support\__init__.py", line 2064 in run_unittest
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\runtest.py", line 178 in test_runner
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\runtest.py", line 182 in runtest_inner
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\runtest.py", line 127 in runtest
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\runtest_mp.py", line 68 in run_tests_worker
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\main.py", line 600 in _main
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\main.py", line 586 in main
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\main.py", line 640 in main
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\regrtest.py", line 46 in _main
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\regrtest.py", line 50 in <module>
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\runpy.py", line 85 in _run_code
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\runpy.py", line 192 in _run_module_as_main
Windows fatal exception: code 0x80000003

Current thread 0x00000de4 (most recent call first):
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\test_mmap.py", line 645 in test_crasher_on_windows
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\case.py", line 642 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\case.py", line 702 in __call__
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 122 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 84 in __call__
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 122 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 84 in __call__
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 122 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\suite.py", line 84 in __call__
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\unittest\runner.py", line 176 in run
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\support\__init__.py", line 1968 in _run_suite
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\support\__init__.py", line 2064 in run_unittest
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\runtest.py", line 178 in test_runner
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\runtest.py", line 182 in runtest_inner
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\runtest.py", line 127 in runtest
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\runtest_mp.py", line 68 in run_tests_worker
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\main.py", line 600 in _main
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\main.py", line 586 in main
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\libregrtest\main.py", line 640 in main
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\regrtest.py", line 46 in _main
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\test\regrtest.py", line 50 in <module>
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\runpy.py", line 85 in _run_code
  File "C:\buildbot.python.org\3.x.kloth-win64\build\lib\runpy.py", line 192 in _run_module_as_main

The bug:
https://github.com/python/cpython/pull/12073/files#r263026496

The bug:
https://github.com/python/cpython/pull/12073/files#r263026496
(correct link, sorry)

--

If nobody comes up quickly with a fix, I will revert the change to repair Windows buildbots: PR 12198, as part of the buildbot policy ("revert on failure"). I don't have the bandwidth to dig into this failure.
https://pythondev.readthedocs.io/ci.html#revert-on-fail

"""
r263026496">https://github.com/python/cpython/pull/12073/files#r263026496
"""

Oh wow, that's really strange. I'm sure that I wrote "https://..." URL but my URL became "r263026496">https://..." !?

3rd attempt to post the link:
https://github.com/python/cpython/pull/12073/files#r263026496

Anyway, my link points to my comment:

"""


PyMem_Free(m_obj->tagname) is called below without holding the GIL: that's illegal. Other move the call to free when the GIL is hold again, or use PyMem_RawFree().

"Warning: The GIL must be held when using these functions. "
https://docs.python.org/dev/c-api/memory.html#memory-interface

Note: You can use PYTHONMALLOC=debug or -X dev to reproduce the issue on a Python compiled in release mode.
"""

On the #ifdef MS_WINDOWS path of mmap_object_dealloc().

Thanks Victor. I think this fixes it https://github.com/python/cpython/pull/12199

BTW should this be backported to 3.7?

New changeset dc078947a5033a048d804e244e847b5844734439 by Victor Stinner (Davide Rizzo) in branch 'master':
bpo-36139: Fix mmap_object_dealloc(): hold the GIL to call PyMem_Free() (GH-12199)
https://github.com/python/cpython/commit/dc078947a5033a048d804e244e847b5844734439

The second commit does fix the regression (I tested manually on Windows with Python compiled in debug mode).

> Oh wow, that's really strange. I'm sure that I wrote "https://..." URL but my URL became "r263026496">https://..." !?

The links are now fixed (Roundup was getting confused by the rNNNNNN, since it looks like a SVN revision).

4th attempt (sorry for the spam, it's just to check the Roundup fix!):
https://github.com/python/cpython/pull/12073/files#r263026496

Yeah! Roundup is fixed, thanks Ezio!

messages: + msg337355
nosy: + ezio.melotti

stage: patch review -> resolved

nosy: + vstinner
messages: + msg337320

resolution: fixed -> (no value)

nosy: + benjamin.peterson
messages: + msg337317

resolution: fixed
stage: patch review -> resolved

messages: + msg336829
versions: + Python 3.7

messages: + msg336827