From: Andres Freund <andres@anarazel.de>
To: Aleksander Alekseev <a.alekseev@postgrespro.ru>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Patch: fix lock contention for HASHHDR.mutex
Date: Tue, 15 Dec 2015 13:25:13 +0100
Message-ID: <20151215122513.GC17724@alap3.anarazel.de> (raw)
In-Reply-To: <20151211170001.78ded9d7@fujitsu>
References: <20151211170001.78ded9d7@fujitsu>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>
On 2015-12-11 17:00:01 +0300, Aleksander Alekseev wrote:
> The problem is that code between LWLockAcquire (lock.c:881) and
> LWLockRelease (lock.c:1020) can _sometimes_ run up to 3-5 ms. Using
> old-good gettimeofday and logging method I managed to find a bottleneck:
>
> -- proclock = SetupLockInTable [lock.c:892]
> `-- proclock = (PROCLOCK *) hash_search_with_hash_value [lock.c:1105]
> `-- currBucket = get_hash_entry(hashp) [dynahash.c:985]
> `-- SpinLockAcquire(&hctl->mutex) [dynahash.c:1187]
>
> If my measurements are not wrong (I didn't place gettimeofday between
> SpinLockAquire/SpinLockRelease, etc) we sometimes spend about 3 ms here
> waiting for a spinlock, which doesn't seems right.
Well, it's quite possible that your process was scheduled out, while
waiting for that spinlock. Which'd make 3ms pretty normal.
I'd consider using a LWLock instead of a spinlock here. I've seen this
contended in a bunch of situations, and the queued behaviour, combined
with directed wakeups on the OS level, ought to improve the worst case
behaviour measurably.
Greetings,
Andres Freund
--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-hackers@postgresql.org
Cc: andres@anarazel.de, a.alekseev@postgrespro.ru
Subject: Re: Patch: fix lock contention for HASHHDR.mutex
In-Reply-To: <20151215122513.GC17724@alap3.anarazel.de>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox