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Re: Odd blocking (or massively latent) issue - even with EXPLAIN

From: Jim Vanns <james(dot)vanns(at)framestore(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Jim Vanns <james(dot)vanns(at)framestore(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Odd blocking (or massively latent) issue - even with EXPLAIN
Date: 2012-07-23 15:49:25
Message-ID: 1343058565.12579.126.camel@sys367.ldn.framestore.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Mon, 2012-07-23 at 11:09 -0400, Tom Lane wrote:
> Jim Vanns <james(dot)vanns(at)framestore(dot)com> writes:
> > We're seeing SELECT statements and even EXPLAIN (no ANAYLZE) 
> > statements hang indefinitely until *something* (we don't know what)
> > releases some kind of resource or no longer becomes a massive bottle
> > neck. These are the symptoms.
> 
> Does anything show up as blocked in the pg_locks view?

Nope.

> Could you attach to the stuck process with gdb and get a stack trace?

Haven't been quite brave enough to do that yet - this is a production
server. I did manage to strace a process though - it (the server side
process of a psql EXPLAIN) appeared to spin on an awful lot of semop()
calls with the occasional read(). Of course, in the context of a shared
memory system such as postgres I'd expect to see quite a lot of semop()
calls but I've no idea how much is normal and how much is excessive.

Jim

> 			regards, tom lane
> 

-- 
Jim Vanns
Systems Programmer
Framestore


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