Re: PATCH: two slab-like memory allocators

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: John Gorman <johngorman2(at)gmail(dot)com>
Cc: Petr Jelinek <petr(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: PATCH: two slab-like memory allocators
Date: 2016-10-05 04:22:19
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attached is v3 of the patches, with a few minor fixes in Slab, and much
larger fixes in GenSlab.

Slab (minor fixes)
- Removed the unnecessary memset() of new blocks in SlabAlloc(),
although we still need to zero the free bitmap at the end of the block.
- Renamed minFreeCount to minFreeChunks, added a few comments explaining
why/how firstFreeChunk and minFreeChunks are maintained.
- Fixed / improved a bunch of additional comments, based on feedback.

Fixed a bunch of bugs that made GenSlab utterly useless. Firstly,
chunkSize was not stored in GenSlabContextCreate(), so this check in

if (size <= set->chunkSize)
return MemoryContextAlloc(set->slab, set->chunkSize);
return MemoryContextAlloc(set->aset, size);

always fell through to the set->aset case, not allocating stuff in the
Slab at all.

Secondly, nallocations / nbytes counters were not updated at all, so the
Slab was never recreated, so GenSlab was not really generational.

This only affected 1 of 3 contexts in ReorderBuffer, but apparently
those "important ones" to affect performance.

Both issues are fixed in the attached v3, which also introduces two
additional improvements discussed in this thread:

- The chunk size is limited by ALLOCSET_SEPARATE_THRESHOLD, as for large
chunks AllocSet works just fine (thanks to keeping them out of free list

- Instead of specifying blockSize and chunkSize, GenSlabCreate() now
accepts three parameters - minBlockSize, minChunkCount and chunkSize,
and computes the minimum block size (>= minBlockSize), sufficient to
store minChunkCount chunks, each chunkSize bytes. This works much better
in the auto-tuning scenario.


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
slab-allocators-v3.tgz application/x-compressed-tar 16.4 KB

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