Feedback-ID: ia2694551:Fastmail
Date: Fri, 31 Jan 2025 15:25:18 +0100
From: =?utf-8?Q?=C3=81lvaro?= Herrera <alvherre@alvh.no-ip.org>
To: Dmitry Koterov <dmitry.koterov@gmail.com>
Cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: hash_search_with_hash_value is high in "perf top" on a replica
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On 2025-Jan-31, Dmitry Koterov wrote:

> PG "startup recovering" eats up a lot of CPU (like 65 %user and 30 %sys),
> which is a little surprising (what is it doing with all those CPU cycles?
> it looked like WAL replay should be more IO bound than CPU bound?).
> 
> Running "perf top -p <pid>", it shows this:
> 
> Samples: 1M of event 'cycles:P', 4000 Hz, Event count (approx.):
> 18178814660 lost: 0/0 drop: 0/0
> Overhead  Shared Object     Symbol
>   16.63%  postgres          [.] hash_search_with_hash_value

Yeah, I noticed that this function was showing high in some profiles a
couple of days ago as well.  Looking now at hashfn.c (hash_bytes_uint32
there is the function involved in the buffer mapping hash table), the
comments state that we're updated to Bob Jenkins code from 2006, but
there's a version in his website that (if I read correctly) is twice as
fast as what we're using now:
http://burtleburtle.net/bob/hash/spooky.html

Apparently this code in our repo is mostly unchanged since commit
1f559b7d3aa4, in 2007.

He mentions that on Intel chips, Google's CityHash is faster; but they
in turn claim that the difference is small on Intel chips and that
Jenkins' hash is better on AMD chips.
https://github.com/google/cityhash

Anyway if you wanted to try your luck at improving things, here's your
chance.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/