Re: hung backends stuck in spinlock heavy endless loop

On 2015-01-14 10:05:01 -0500, Tom Lane wrote:
> Merlin Moncure <mmoncure(at)gmail(dot)com> writes:
> > On Wed, Jan 14, 2015 at 8:41 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> What are the autovac processes doing (according to pg_stat_activity)?
>
> > pid,running,waiting,query
> > 7105,00:28:40.789221,f,autovacuum: VACUUM ANALYZE pg_catalog.pg_class

It'd be interesting to know whether that vacuum gets very frequent
semaphore wakeups. Could you strace it for a second or three?

How did this perform < 9.4? Can you guess how many times these dynamic
statements are planned? How many different relations are accessed in the
dynamically planned queries?

> Hah, I suspected as much. Is that the one that's stuck in
> LockBufferForCleanup, or the other one that's got a similar backtrace
> to all the user processes?

Do you have a theory? Right now it primarily looks like contention on a
single buffer due to the high number of dynamic statements, possibly
made worse by the signalling between normal pinners and vacuum waiting
for cleanup.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services