Re: hung backends stuck in spinlock heavy endless loop

On 2015-01-14 10:13:32 -0500, Tom Lane wrote:
> Merlin Moncure <mmoncure@gmail.com> writes:
> > Yes, it is pg_class is coming from  LockBufferForCleanup ().  As you
> > can see above, it has a shorter runtime.   So it was killed off once
> > about a half hour ago which did not free up the logjam.  However, AV
> > spawned it again and now it does not respond to cancel.
> 
> Interesting.  That seems like there might be two separate issues at
> play.  It's plausible that LockBufferForCleanup might be interfering
> with other attempts to scan the index, but then why wouldn't killing
> the AV have unstuck things?

LockBufferForCleanup() unfortunately isn't interruptible. I've every now
and then seen vacuums being stuck for a long while, trying to acquire
cleanup locks - IIRC I complained about that on list even.  So autovac
will only be cancelled when going to the next page.  And even if the
page ever gets a zero (well, one) refcount, by the time autovac is woken
up via the semaphore, it'll often end up being used again.

Greetings,

Andres Freund

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