author | Shane McErlean <shane.mcerlean@accenture.com> |
Fri, 27 Aug 2010 15:33:49 +0100 | |
changeset 101 | 1e82a8be084a |
parent 1 | 2fb8b9db1c86 |
permissions | -rw-r--r-- |
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This document describes some caveats about the use of Valgrind with |
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Python. Valgrind is used periodically by Python developers to try |
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to ensure there are no memory leaks or invalid memory reads/writes. |
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If you don't want to read about the details of using Valgrind, there |
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are still two things you must do to suppress the warnings. First, |
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you must use a suppressions file. One is supplied in |
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Misc/valgrind-python.supp. Second, you must do one of the following: |
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* Uncomment Py_USING_MEMORY_DEBUGGER in Objects/obmalloc.c, |
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then rebuild Python |
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* Uncomment the lines in Misc/valgrind-python.supp that |
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suppress the warnings for PyObject_Free and PyObject_Realloc |
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If you want to use Valgrind more effectively and catch even more |
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memory leaks, you will need to configure python --without-pymalloc. |
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PyMalloc allocates a few blocks in big chunks and most object |
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allocations don't call malloc, they use chunks doled about by PyMalloc |
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from the big blocks. This means Valgrind can't detect |
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many allocations (and frees), except for those that are forwarded |
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to the system malloc. Note: configuring python --without-pymalloc |
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makes Python run much slower, especially when running under Valgrind. |
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You may need to run the tests in batches under Valgrind to keep |
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the memory usage down to allow the tests to complete. It seems to take |
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about 5 times longer to run --without-pymalloc. |
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Apr 15, 2006: |
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test_ctypes causes Valgrind 3.1.1 to fail (crash). |
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test_socket_ssl should be skipped when running valgrind. |
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The reason is that it purposely uses uninitialized memory. |
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This causes many spurious warnings, so it's easier to just skip it. |
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Details: |
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-------- |
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Python uses its own small-object allocation scheme on top of malloc, |
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called PyMalloc. |
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Valgrind may show some unexpected results when PyMalloc is used. |
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Starting with Python 2.3, PyMalloc is used by default. You can disable |
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PyMalloc when configuring python by adding the --without-pymalloc option. |
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If you disable PyMalloc, most of the information in this document and |
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the supplied suppressions file will not be useful. As discussed above, |
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disabling PyMalloc can catch more problems. |
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If you use valgrind on a default build of Python, you will see |
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many errors like: |
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==6399== Use of uninitialised value of size 4 |
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==6399== at 0x4A9BDE7E: PyObject_Free (obmalloc.c:711) |
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==6399== by 0x4A9B8198: dictresize (dictobject.c:477) |
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These are expected and not a problem. Tim Peters explains |
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the situation: |
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PyMalloc needs to know whether an arbitrary address is one |
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that's managed by it, or is managed by the system malloc. |
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The current scheme allows this to be determined in constant |
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time, regardless of how many memory areas are under pymalloc's |
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control. |
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The memory pymalloc manages itself is in one or more "arenas", |
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each a large contiguous memory area obtained from malloc. |
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The base address of each arena is saved by pymalloc |
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in a vector. Each arena is carved into "pools", and a field at |
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the start of each pool contains the index of that pool's arena's |
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base address in that vector. |
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Given an arbitrary address, pymalloc computes the pool base |
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address corresponding to it, then looks at "the index" stored |
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near there. If the index read up is out of bounds for the |
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vector of arena base addresses pymalloc maintains, then |
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pymalloc knows for certain that this address is not under |
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pymalloc's control. Otherwise the index is in bounds, and |
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pymalloc compares |
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the arena base address stored at that index in the vector |
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to |
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the arbitrary address pymalloc is investigating |
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pymalloc controls this arbitrary address if and only if it lies |
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in the arena the address's pool's index claims it lies in. |
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It doesn't matter whether the memory pymalloc reads up ("the |
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index") is initialized. If it's not initialized, then |
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whatever trash gets read up will lead pymalloc to conclude |
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(correctly) that the address isn't controlled by it, either |
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because the index is out of bounds, or the index is in bounds |
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but the arena it represents doesn't contain the address. |
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This determination has to be made on every call to one of |
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pymalloc's free/realloc entry points, so its speed is critical |
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(Python allocates and frees dynamic memory at a ferocious rate |
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-- everything in Python, from integers to "stack frames", |
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lives in the heap). |