Re: MultiXact\SLRU buffers configuration

From: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
To: i(dot)lazarev(at)postgrespro(dot)ru
Cc: Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru>, Andres Freund <andres(at)anarazel(dot)de>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Gilles Darold <gilles(at)darold(dot)net>, Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Alexander Korotkov <aekorotkov(at)gmail(dot)com>, Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: MultiXact\SLRU buffers configuration
Date: 2022-08-18 03:35:15
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> On 17 Aug 2022, at 00:36, i(dot)lazarev(at)postgrespro(dot)ru wrote:
>> Andrey Borodin wrote 2022-07-23 11:39:
>> Yura, thank you for your benchmarks!
>> We already knew that patch can save the day on pathological workloads,
>> now we have a proof of this.
>> Also there's the evidence that user can blindly increase size of SLRU
>> if they want (with the second patch). So there's no need for hard
>> explanations on how to tune the buffers size.
> Hi @Andrey.Borodin, With some considerations and performance checks from @Yura.Sokolov we simplified your approach by the following:
> 1. Preamble. We feel free to increase any SLRU's, since there's no performance degradation on large Buffers count using your SLRU buckets solution.
> 2. `slru_buffers_size_scale` is only one config param introduced for all SLRUs. It scales SLRUs upper cap by power 2.
> 3. All SLRU buffers count are capped by both `MBuffers (shared_buffers)` and `slru_buffers_size_scale`. see
> 4. Magic initial constants `NUM_*_BUFFERS << slru_buffers_size_scale` are applied for every SLRU.
> 5. All SLRU buffers are always sized as power of 2, their hash bucket size is always 8.
> There's attached patch for your consideration. It does gather and simplify both `v21-0001-Make-all-SLRU-buffer-sizes-configurable.patch` and `v21-0002-Divide-SLRU-buffers-into-8-associative-banks.patch` to much simpler approach.

I like the idea of one knob instead of one per each SLRU. Maybe we even could deduce sane value from NBuffers? That would effectively lead to 0 knobs :)

Your patch have a prefix "v22-0006", does it mean there are 5 previous steps of the patchset?

Thank you!

Best regards, Andrey Borodin.

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