Re: Heap truncation without AccessExclusiveLock (9.4)

Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, May 15, 2013 at 11:35 AM, Heikki Linnakangas
> <hlinnakangas@vmware.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