Re: slow startup due to LWLockAssign() spinlock

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Andres Freund <andres(at)2ndquadrant(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: slow startup due to LWLockAssign() spinlock
Date: 2014-04-16 23:33:52
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On Tue, Feb 4, 2014 at 12:58:49AM +0100, Andres Freund wrote:
> On 2014-02-03 11:22:45 -0500, Tom Lane wrote:
> > 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 ...
> So, here's a flag implementing things with that flag. I kept your name,
> as it's more in line with ipci.c's naming, but it looks kinda odd
> besides proc_exit_inprogress.

Uh, where are we on this?

Bruce Momjian <bruce(at)momjian(dot)us>

+ Everyone has their own god. +

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