| This document describes some caveats about the use of Valgrind with |
| Python. Valgrind is used periodically by Python developers to try |
| to ensure there are no memory leaks or invalid memory reads/writes. |
| |
| If you want to enable valgrind support in Python, you will need to |
| configure Python --with-valgrind option or an older option |
| --without-pymalloc. |
| |
| UPDATE: Python 3.6 now supports PYTHONMALLOC=malloc environment variable which |
| can be used to force the usage of the malloc() allocator of the C library. |
| |
| If you don't want to read about the details of using Valgrind, there |
| are still two things you must do to suppress the warnings. First, |
| you must use a suppressions file. One is supplied in |
| Misc/valgrind-python.supp. Second, you must uncomment the lines in |
| Misc/valgrind-python.supp that suppress the warnings for PyObject_Free and |
| PyObject_Realloc. |
| |
| If you want to use Valgrind more effectively and catch even more |
| memory leaks, you will need to configure python --without-pymalloc. |
| PyMalloc allocates a few blocks in big chunks and most object |
| allocations don't call malloc, they use chunks doled about by PyMalloc |
| from the big blocks. This means Valgrind can't detect |
| many allocations (and frees), except for those that are forwarded |
| to the system malloc. Note: configuring python --without-pymalloc |
| makes Python run much slower, especially when running under Valgrind. |
| You may need to run the tests in batches under Valgrind to keep |
| the memory usage down to allow the tests to complete. It seems to take |
| about 5 times longer to run --without-pymalloc. |
| |
| Apr 15, 2006: |
| test_ctypes causes Valgrind 3.1.1 to fail (crash). |
| test_socket_ssl should be skipped when running valgrind. |
| The reason is that it purposely uses uninitialized memory. |
| This causes many spurious warnings, so it's easier to just skip it. |
| |
| |
| Details: |
| -------- |
| Python uses its own small-object allocation scheme on top of malloc, |
| called PyMalloc. |
| |
| Valgrind may show some unexpected results when PyMalloc is used. |
| Starting with Python 2.3, PyMalloc is used by default. You can disable |
| PyMalloc when configuring python by adding the --without-pymalloc option. |
| If you disable PyMalloc, most of the information in this document and |
| the supplied suppressions file will not be useful. As discussed above, |
| disabling PyMalloc can catch more problems. |
| |
| PyMalloc uses 256KB chunks of memory, so it can't detect anything |
| wrong within these blocks. For that reason, compiling Python |
| --without-pymalloc usually increases the usefulness of other tools. |
| |
| If you use valgrind on a default build of Python, you will see |
| many errors like: |
| |
| ==6399== Use of uninitialised value of size 4 |
| ==6399== at 0x4A9BDE7E: PyObject_Free (obmalloc.c:711) |
| ==6399== by 0x4A9B8198: dictresize (dictobject.c:477) |
| |
| These are expected and not a problem. Tim Peters explains |
| the situation: |
| |
| PyMalloc needs to know whether an arbitrary address is one |
| that's managed by it, or is managed by the system malloc. |
| The current scheme allows this to be determined in constant |
| time, regardless of how many memory areas are under pymalloc's |
| control. |
| |
| The memory pymalloc manages itself is in one or more "arenas", |
| each a large contiguous memory area obtained from malloc. |
| The base address of each arena is saved by pymalloc |
| in a vector. Each arena is carved into "pools", and a field at |
| the start of each pool contains the index of that pool's arena's |
| base address in that vector. |
| |
| Given an arbitrary address, pymalloc computes the pool base |
| address corresponding to it, then looks at "the index" stored |
| near there. If the index read up is out of bounds for the |
| vector of arena base addresses pymalloc maintains, then |
| pymalloc knows for certain that this address is not under |
| pymalloc's control. Otherwise the index is in bounds, and |
| pymalloc compares |
| |
| the arena base address stored at that index in the vector |
| |
| to |
| |
| the arbitrary address pymalloc is investigating |
| |
| pymalloc controls this arbitrary address if and only if it lies |
| in the arena the address's pool's index claims it lies in. |
| |
| It doesn't matter whether the memory pymalloc reads up ("the |
| index") is initialized. If it's not initialized, then |
| whatever trash gets read up will lead pymalloc to conclude |
| (correctly) that the address isn't controlled by it, either |
| because the index is out of bounds, or the index is in bounds |
| but the arena it represents doesn't contain the address. |
| |
| This determination has to be made on every call to one of |
| pymalloc's free/realloc entry points, so its speed is critical |
| (Python allocates and frees dynamic memory at a ferocious rate |
| -- everything in Python, from integers to "stack frames", |
| lives in the heap). |