Horton

Re: Reducing the chunk header sizes on all memory context types

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: David Rowley <dgrowleyml@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2022-09-01T00:23:41Z
Lists: pgsql-hackers 
Tomas Vondra <tomas.vondra@enterprisedb.com> writes:
> Focusing on the aset, vast majority of allocations (60M out of 64M) is
> small enough to use power-of-2 logic, and we go from 6.3GB to 8.2GB, so
> ~30%. Not great, not terrible.

Not sure why this escaped me before, but I remembered another argument
for not forcibly adding space for a sentinel: if you don't have room,
that means the chunk end is up against the header for the next chunk,
which means that any buffer overrun will clobber that header.  So we'll
detect the problem anyway if we validate the headers to a reasonable
extent.

The hole in this argument is that the very last chunk allocated in a
block might have no following chunk to validate.  But we could probably
special-case that somehow, maybe by laying down a sentinel in the free
space, where it will get overwritten by the next chunk when that does
get allocated.

30% memory bloat seems like a high price to pay if it's adding negligible
detection ability, which it seems is true if this argument is valid.
Is there reason to think we can't validate headers enough to catch
clobbers?

			regards, tom lane

Commits

  1. Harden memory context allocators against bogus chunk pointers.

  2. Improve our ability to detect bogus pointers passed to pfree et al.

  3. Remove MemoryContextContains().

  4. Remove uses of MemoryContextContains in nodeAgg.c and nodeWindowAgg.c.

  5. Temporarily make MemoryContextContains return false

  6. Make MemoryContextContains work correctly again

  7. Make more effort to put a sentinel at the end of allocated memory

  8. Fix some possibly latent bugs in slab.c

  9. Various cleanups of the new memory context header code

  10. Revert "Add missing padding from MemoryChunk struct"

  11. Use MAXALIGN() in calculations using sizeof(SlabBlock)

  12. Add missing padding from MemoryChunk struct

  13. Improve performance of and reduce overheads of memory management