When PYTHONMALLOC=debug environment variable or -X dev command line option is used, Python installs debug hooks on memory allocators which add 2 size_t before and 2 size_t after each memory block: it adds 32 bytes to every memory allocation.I'm debugging crashes and memory leaks in CPython for 10 years, and I simply never had to use "serialno". So I simply propose attached pull request to remove it to reduce the memory footprint: I measured a reduction around -5% (ex: 1.2 MiB on 33.0 MiB when running test_asyncio). A smaller memory footprint allows to use this feature on devices with small memory, like embedded devices.The change also fix race condition in debug memory allocators:bpo-31473, "Debug hooks on memory allocators are not thread safe (serialno variable)".Using tracemalloc, it is already possible (since Python 3.6) to find where a memory block has been allocated, and so decide where to put a breakpoint when debugging.If someone cares about the "serialno" field, maybe we can keep code using a compilation flag, like a C #define."serialno" is documented as: "an excellent way to set a breakpoint on the next run, to capture the instant at which this block was passed out." But again, I never used it...--Some examples of the *peak* memory usage without => with the change:* -c pass: 2321.8 kB => 2437.1 kB (-115.3 kiB, -5%)* -m test test_os test_sys: 14252.3 kB => 13598.6 kB (-653.7 kiB, -5%)* -m test test_asyncio: 34194.2 kB => 32963.1 kB (-1231.1 kiB, -4%)Command used to measure the memory consumption:$ ./python -i -X tracemalloc -c pass>>> import tracemalloc; print("%.1f kB" % (tracemalloc.get_traced_memory()[1] / 1024.))With the patch:diff --git a/Modules/_tracemalloc.c b/Modules/_tracemalloc.cindexc5d5671032..e010c2ef84 100644--- a/Modules/_tracemalloc.c+++ b/Modules/_tracemalloc.c@@ -582,6 +582,8 @@ tracemalloc_add_trace(unsigned int domain, uintptr_t ptr,     _Py_hashtable_entry_t* entry;     int res; +    size += 4 * sizeof(size_t);+     assert(_Py_tracemalloc_config.tracing);      traceback = traceback_new();Replace 4 with 3 to measure memory used with the change.--Since Python 3.6, when the debug memory allocator detects a bug (ex: buffer overflow), it now also displays the Python traceback where the memory block has been allocated if tracemalloc is tracing Python memory allocations.Example with buffer_overflow.py:---import _testcapidef func():    _testcapi.pymem_buffer_overflow()def main():    func()if __name__ == "__main__":    main()---Output:---$ ./python -X tracemalloc=10 -X dev bug.pyDebug memory block at address p=0x7f45e85c3270: API 'm'    16 bytes originally requested    The 7 pad bytes at p-7 are FORBIDDENBYTE, as expected.    The 8 pad bytes at tail=0x7f45e85c3280 are not all FORBIDDENBYTE (0xfd):        at tail+0: 0x78 *** OUCH        at tail+1: 0xfd        at tail+2: 0xfd        at tail+3: 0xfd        at tail+4: 0xfd        at tail+5: 0xfd        at tail+6: 0xfd        at tail+7: 0xfd    Data at p: cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cdMemory block allocated at (most recent call first):  File "bug.py", line 4  File "bug.py", line 7  File "bug.py", line 10Fatal Python error: bad trailing pad byteCurrent thread 0x00007f45f5660740 (most recent call first):  File "bug.py", line 4 in func  File "bug.py", line 7 in main  File "bug.py", line 10 in <module>Aborted (core dumped)---The interesting part is "Memory block allocated at (most recent call first):". Traceback reconstructed manually:---Memory block allocated at (most recent call first):  File "bug.py", line 4    _testcapi.pymem_buffer_overflow()  File "bug.py", line 7    func()  File "bug.py", line 10    main()---You can see exactly where the memory block has been allocated.Note: Internally, the _PyTraceMalloc_GetTraceback() function is used to get the traceback where a memory block has been allocated.--Extract of _PyMem_DebugRawAlloc() inObjects/obmalloc.c:/* Let S = sizeof(size_t).  The debug malloc asks for 4*S extra bytes and   fills them with useful stuff, here calling the underlying malloc's result p:p[0: S]    Number of bytes originally asked for.  This is a size_t, big-endian (easier    to read in a memory dump).p[S]    API ID.  SeePEP 445.  This is a character, but seems undocumented.p[S+1: 2*S]    Copies of FORBIDDENBYTE.  Used to catch under- writes and reads.p[2*S: 2*S+n]    The requested memory, filled with copies of CLEANBYTE.    Used to catch reference to uninitialized memory.    &p[2*S] is returned.  Note that this is 8-byte aligned if pymalloc    handled the request itself.p[2*S+n: 2*S+n+S]    Copies of FORBIDDENBYTE.  Used to catch over- writes and reads.p[2*S+n+S: 2*S+n+2*S]    A serial number, incremented by 1 on each call to _PyMem_DebugMalloc    and _PyMem_DebugRealloc.    This is a big-endian size_t.    If "bad memory" is detected later, the serial number gives an    excellent way to set a breakpoint on the next run, to capture the    instant at which this block was passed out.*//* Layout: [SSSS IFFF CCCC...CCCC FFFF NNNN] *          ^--- p    ^--- data   ^--- tail   S: nbytes stored as size_t   I: API identifier (1 byte)   F: Forbidden bytes (size_t - 1 bytes before, size_t bytes after)   C: Clean bytes used later to store actual data   N: Serial number stored as size_t */The last size_t written at the end of each memory block is "serialno". It is documented as: "an excellent way to set a breakpoint on the next run, to capture the instant at which this block was passed out."

I wrote 2 pull requests for the two options:*PR 12795 removes serialno field*PR 12796 adds PYMEM_DEBUG_SERIALNO define which is unset by default

While testing my changes, I found a bug in test_sys:./python -X tracemalloc -m test test_sys -v -m test_getallocatedblocks======================================================================ERROR: test_getallocatedblocks (test.test_sys.SysModuleTest)----------------------------------------------------------------------Traceback (most recent call last):  File "/home/vstinner/prog/python/master/Lib/test/test_sys.py", line 770, in test_getallocatedblocks    alloc_name = _testcapi.pymem_getallocatorsname()RuntimeError: cannot get allocators nameAttachedPR 12797 fix it.

I never used the serialno too.

This issue is related to the following thread on python-dev which discuss disabling Py_TRACE_REFS by default,bpo-36465, to reduce the memory footprint in debug mode:https://mail.python.org/pipermail/python-dev/2019-April/157015.html

The serialno has been added at the same time than the whole debug hooks on Python memory allocated by Tim Peters in 2002, 17 years ago:commitddea208be9e2a8fa281e25ebbc890378dd2aa286Author: Tim Peters <tim.peters@gmail.com>Date:   Sat Mar 23 10:03:50 2002 +0000    Give Python a debug-mode pymalloc, much as sketched on Python-Dev.        When WITH_PYMALLOC is defined, define PYMALLOC_DEBUG to enable the debug    allocator.  This can be done independent of build type (release or debug).    A debug build automatically defines PYMALLOC_DEBUG when pymalloc is    enabled.  It's a detected error to define PYMALLOC_DEBUG when pymalloc    isn't enabled.        Two debugging entry points defined only under PYMALLOC_DEBUG:        + _PyMalloc_DebugCheckAddress(const void *p) can be used (e.g., from gdb)      to sanity-check a memory block obtained from pymalloc.  It sprays      info to stderr (see next) and dies via Py_FatalError if the block is      detectably damaged.        + _PyMalloc_DebugDumpAddress(const void *p) can be used to spray info      about a debug memory block to stderr.        A tiny start at implementing "API family" checks isn't good for    anything yet.        _PyMalloc_DebugRealloc() has been optimized to do little when the new    size is <= old size.  However, if the new size is larger, it really    can't call the underlying realloc() routine without either violating its    contract, or knowing something non-trivial about how the underlying    realloc() works.  A memcpy is always done in this case.        This was a disaster for (and only) one of the std tests:  test_bufio    creates single text file lines up to a million characters long.  On    Windows, fileobject.c's get_line() uses the horridly funky    getline_via_fgets(), which keeps growing and growing a string object    hoping to find a newline.  It grew the string object 1000 bytes each    time, so for a million-character string it took approximately forever    (I gave up after a few minutes).        So, also:        fileobject.c, getline_via_fgets():  When a single line is outrageously    long, grow the string object at a mildly exponential rate, instead of    just 1000 bytes at a time.        That's enough so that a debug-build test_bufio finishes in about 5 seconds    on my Win98SE box.  I'm curious to try this on Win2K, because it has very    different memory behavior than Win9X, and test_bufio always took a factor    of 10 longer to complete on Win2K.  It *could* be that the endless    reallocs were simply killing it on Win2K even in the release build.

New changeset9b8314cfe29ca532fc335277f6c36b72e6132922 by Victor Stinner in branch 'master':bpo-36611: Fix test_sys.test_getallocatedblocks() (GH-12797)https://github.com/python/cpython/commit/9b8314cfe29ca532fc335277f6c36b72e6132922

New changeset7182e653fb5c6f78f05892b6ed302fc8db8978d3 by Miss Islington (bot) in branch '3.7':bpo-36611: Fix test_sys.test_getallocatedblocks() (GH-12797)https://github.com/python/cpython/commit/7182e653fb5c6f78f05892b6ed302fc8db8978d3

New changesete8f9acf03484c6c3f163f04a76321419369c28aa by Victor Stinner in branch 'master':bpo-36611: Disable serialno field of debug memory allocators (#12796)https://github.com/python/cpython/commit/e8f9acf03484c6c3f163f04a76321419369c28aa

We decided to only disable the code by default, but the code stays until we are sure that nobody uses it.