Re: Seq scans status update

On Mon, 2007-05-28 at 17:36 -0400, Tom Lane wrote:
> Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
> > One idea is to keep track which pins are taken using the bulk strategy.
> > It's a bit tricky when a buffer is pinned multiple times since we don't
> > know which ReleaseBuffer corresponds which ReadBuffer, but perhaps we
> > could get away with just a flag per pinned buffer. Set the flag when a
> > buffer is pinned with bulk strategy and it wasn't pinned by us before,
> > and clear it when it's pinned with another strategy. I'm thinking we
> > steal one bit from PrivateRefCount for this.
>
> Seems like a mess. Why don't we just fix it so there's no need for
> different behavior at Unpin time? The facts on the ground are that
> the current patch's change in UnpinBuffer is a no-op anyway, because
> of the tupletableslot interference.
>
> The behavior I'm imagining is just that when we try to take a buffer
> from the ring, if its usage count exceeds 1 then drop it from the ring
> and get another buffer. 1 would be the expected case if no one had
> touched it since we last used it.
>
> >> A heapscan would pin the buffer only once and hence bump its count at
> >> most once, so I don't see a big problem here. Also, I'd argue that
> >> buffers that had a positive usage_count shouldn't get sucked into the
> >> ring to begin with.
>
> > True, except that with the synchronized scans patch two synchronized
> > scans will pin the buffer twice.
>
> Hmm. But we probably don't want the same buffer in two different
> backends' rings, either. You *sure* the sync-scan patch has no
> interaction with this one?
>

I will run some tests again tonight, I think the interaction needs more
testing than I did originally. Also, I'm not sure that the hardware I
have is sufficient to test those cases.

It looks like the case to worry about is when there are a large number
of scans on the same table and the I/O system is fast enough that it
causes lock contention on the buffers in the rings. Is this the case
you're worried about?

Also, keep in mind that I have added a SyncScanLock after I ran those
tests. That could have an effect.

Regards,
Jeff Davis