Date: Fri, 18 Dec 2015 13:04:54 +0300
From: Aleksander Alekseev <a.alekseev@postgrespro.ru>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Patch: fix lock contention for HASHHDR.mutex
Message-ID: <20151218130454.391e44df@fujitsu>
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> This idea can improve the situation with ProcLock hash table, but I
> think IIUC what Andres is suggesting would reduce the contention
> around dynahash freelist and can be helpful in many more situations
> including BufMapping locks.

I agree. But as I understand PostgreSQL community doesn't generally
=D0=B0ppr=D0=BEv=D0=B5 big changes that affects whole system. Especially if=
 original
problem was only in one particular place. Therefore for now I suggest
only a small change. Naturally if it will be accepted there is no
reason not to apply same changes for BufMapping or even dynahash itself
with corresponding PROCLOCK hash refactoring.

BTW could you (or anyone?) please help me find this thread regarding
BufMapping or perhaps provide a benchmark? I would like to reproduce
this issue but I can't find anything relevant in a mailing list. Also
it seems to be a good idea to compare alternative approaches that were
mentioned (atomics ops, group leader). Are there any discussions,
benchmarks or patches regarding this topic?

Frankly I have serious doubts regarding atomics ops since they will more
likely create the same contention that a spinlock does. But perhaps
there is a patch that works not the way I think it could work.


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