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: Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, David Rowley <dgrowleyml@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2022-08-09T21:23:00Z
Lists: pgsql-hackers 
Andres Freund <andres@anarazel.de> writes:
> On 2022-08-09 15:21:57 -0400, Tom Lane wrote:
>> Do we really need it to be that tight?  I know we only have 3 methods today,
>> but 8 doesn't seem that far away.  If there were six bits reserved for
>> this I'd be happier.

> We only have so many bits available, so that'd have to come from some other
> resource.  The current division is:

> + * 1.	3-bits to indicate the MemoryContextMethodID
> + * 2.	1-bit to indicate if the chunk is externally managed (see below)
> + * 3.	30-bits for the amount of memory which was reserved for the chunk
> + * 4.	30-bits for the number of bytes that must be subtracted from the chunk
> + *		to obtain the address of the block that the chunk is stored on.

> I suspect we could reduce 3) here a bit, which I think would end up with slab
> context's max chunkSize shrinking further. Which should still be fine.

Hmm, I suppose you mean we could reduce 4) if we needed to.  Yeah, that
seems like a reasonable place to buy more bits later if we run out of
MemoryContextMethodIDs.  Should be fine then.

			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