Hi,
I've been annoyed at the amount of memory used by the backend local
PrivateRefCount array for a couple of reasons:
a) The performance impact of AtEOXact_Buffers() on Assert() enabled
builds is really, really annoying.
b) On larger nodes, the L1/2/3 cache impact of randomly accessing
several megabyte big array at a high frequency is noticeable. I've
seen the access to that to be the primary (yes, really) source of
pipeline stalls.
c) On nodes with significant shared_memory the sum of the per-backend
arrays is a significant amount of memory, that could very well be
used more beneficially.
So what I have done in the attached proof of concept is to have a small
(8 currently) array of (buffer, pincount) that's searched linearly when
the refcount of a buffer is needed. When more than 8 buffers are pinned
a hashtable is used to lookup the values.
That seems to work fairly well. On the few tests I could run on my
laptop - I've done this during a flight - it's a small performance win
in all cases I could test. While saving a fair amount of memory.
Alternatively we could just get rid of the idea of tracking this per
backend, relying on tracking via resource managers...
Greetings,
Andres Freund
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Andres Freund http://www.2ndQuadrant.com/
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