Re: slow startup due to LWLockAssign() spinlock

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: slow startup due to LWLockAssign() spinlock
Date: 2014-02-03 16:22:45
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Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On larger, multi-socket, machines, startup takes a fair bit of time. As
> I was profiling anyway I looked into it and noticed that just about all
> of it is spent in LWLockAssign() called by InitBufferPool(). Starting
> with shared_buffers=48GB on the server Nate Boley provided, takes about
> 12 seconds. Nearly all of it spent taking the ShmemLock spinlock.
> Simply modifying LWLockAssign() to not take the spinlock when
> !IsUnderPostmaster speeds it up to 2 seconds. While certainly not making
> LWLockAssign() prettier it seems enough of a speedup to be worthwile
> nonetheless.

Hm. This patch only works if the postmaster itself never assigns any
LWLocks except during startup. That's *probably* all right, but it
seems a bit scary. Is there any cheap way to make the logic actually
be what your comment claims, namely "Interlocking is not necessary during
postmaster startup"? I guess we could invent a ShmemInitInProgress global
flag ...

regards, tom lane

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