From: | Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
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To: | Andrey Borodin <x4mmm(at)yandex-team(dot)ru> |
Cc: | 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: | 2021-03-26 20:26:40 |
Message-ID: | CA+hUKGKVqrxOp82zER1=XN=yPwV_-OCGAg=ez=1iz9rG+A7Smw@mail.gmail.com |
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Lists: | pgsql-hackers |
On Sat, Mar 27, 2021 at 4:52 AM Andrey Borodin <x4mmm(at)yandex-team(dot)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).
Attachment | Content-Type | Size |
---|---|---|
v13-0001-Make-all-SLRU-buffer-sizes-configurable.patch | application/x-patch | 20.6 KB |
v13-0002-Make-simplehash-easy-to-use-in-shmem.patch | application/x-patch | 5.9 KB |
v13-0003-Support-intrusive-status-flag-in-simplehash.patch | application/x-patch | 5.7 KB |
v13-0004-Add-buffer-mapping-table-for-SLRUs.patch | application/x-patch | 8.5 KB |
v13-0005-fixup-use-simplehash-instead-of-dynahash.patch | application/x-patch | 4.6 KB |
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