Re: Seq scans status update

On Tue, 2007-05-29 at 17:43 -0700, Jeff Davis wrote:
> > 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.
>

I ran some sanity tests last night with cvs head, plus my syncscan20-
cvshead.patch, plus scan_recycle_buffers.v3.patch.

It passed the sanity tests at least.

I did see that there was more interference with sync_seqscan_threshold=0
(always on) and scan_recycle_buffers=0 (off) than I had previously seen
with 8.2.4, so I will test again against 8.2.4 to see why that might be.
The interference that I saw was still quite small, a scan moving
concurrently with 9 other scans was about 10% slower than a scan running
alone -- which is still very good compared with plain cvs head and no
sync scan -- it's just not ideal.

However, turning scan_recycle_buffers between 0 (off), 16, 32, and 128
didn't have much effect. At 32 it appeared to be about 1% worse during
10 scans, but that may have been noise. The other values I tried didn't
have any difference that I could see.

This was really just a quick sanity test, I think more hard data would
be useful.

Regards,
    Jeff Davis