From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Heap truncation without AccessExclusiveLock (9.4) |
Date: | 2013-05-15 23:10:16 |
Message-ID: | 24122.1368659416@sss.pgh.pa.us |
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Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, May 15, 2013 at 11:35 AM, Heikki Linnakangas
> <hlinnakangas(at)vmware(dot)com> wrote:
>> To not slow down common backend
>> operations, the values (or lack thereof) are cached in relcache. To sync the
>> relcache when the values change, there will be a new shared cache
>> invalidation event to force backends to refresh the cached watermark values.
> AFAIK, the sinval mechanism isn't really well-designed to ensure that
> these kinds of notifications arrive in a timely fashion.
Yeah; currently it's only meant to guarantee that you see updates that
were protected by obtaining a heavyweight lock with which your own lock
request conflicts. It will *not* work for the usage Heikki proposes,
at least not without sprinkling sinval queue checks into a lot of places
where they aren't now. And as you say, the side-effects of that would
be worrisome.
> Another problem is that sinval resets are bad for performance, and
> anything we do that pushes more messages through sinval will increase
> the frequency of resets.
I've been thinking that we should increase the size of the sinval ring;
now that we're out from under SysV shmem size limits, it wouldn't be
especially painful to do that. That's not terribly relevant to this
issue though. I agree that we don't want an sinval message per relation
extension, no matter what the ring size is.
regards, tom lane
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