Horton

Re: MultiXact\SLRU buffers configuration

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: Gilles Darold <gilles@darold.net>, Tomas Vondra <tomas.vondra@enterprisedb.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Alexander Korotkov <aekorotkov@gmail.com>, Anastasia Lubennikova <a.lubennikova@postgrespro.ru>, Daniel Gustafsson <daniel@yesql.se>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2021-03-26T20:26:40Z
Lists: pgsql-hackers

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On Sat, Mar 27, 2021 at 4:52 AM Andrey Borodin <x4mmm@yandex-team.ru> wrote:
> Some thoughts on HashTable patch:
> 1. Can we allocate bigger hashtable to reduce probability of collisions?

Yeah, good idea, might require some study.

> 2. Can we use specialised hashtable for this case? I'm afraid hash_search() does comparable number of CPU cycles as simple cycle from 0 to 128. We could inline everything and avoid hashp->hash(keyPtr, hashp->keysize) call. I'm not insisting on special hash though, just an idea.

I tried really hard to not fall into this rabbit h.... [hack hack
hack], OK, here's a first attempt to use simplehash, Andres's
steampunk macro-based robinhood template that we're already using for
several other things, and murmurhash which is inlineable and
branch-free.  I had to tweak it to support "in-place" creation and
fixed size (in other words, no allocators, for use in shared memory).
Then I was annoyed that I had to add a "status" member to our struct,
so I tried to fix that.  Definitely needs more work to think about
failure modes when running out of memory, how much spare space you
need, etc.

I have not experimented with this much beyond hacking until the tests
pass, but it *should* be more efficient...

> 3. pageno in SlruMappingTableEntry seems to be unused.

It's the key (dynahash uses the first N bytes of your struct as the
key, but in this new simplehash version it's more explicit).

Commits

  1. Rework new SLRU test with injection points

  2. injection_point: Add injection_points.stats

  3. injection_points: Add initialization of shmem state when loading module

  4. Add injection-point test for new multixact CV usage

  5. Use conditional variable to wait for next MultiXact offset

  6. Improve performance of subsystems on top of SLRU

  7. Adjust VACUUM hastup LP_REDIRECT comments.

  8. Test replay of regression tests, attempt II.