Tom Lane wrote:
Heikki Linnakangas <[EMAIL PROTECTED]> writes:
I've completed a set of performance tests on a test server. The server has 4 GB of RAM, of which 1 GB is used for shared_buffers.

Perhaps I'm misreading it, but these tests seem to show no improvement
worth spending any effort on --- some of the tests improve a bit but
many get worse, and that's for tests allegedly designed to highlight the
improvement; there's been no attempt to measure the distributed cost of
the additional logic in scenarios where it's not helpful.  To say
nothing of the likelihood that it will be flat-out counterproductive
in yet other scenarios.

Yeah, possibly. I've been thinking hard of scenarios where this would be counterproductive, bulk delete is one that the original patch hurt, but I think I have all interesting scenarios covered now.

Should we not just reject the patch and move on to something more

If Luke is right, the L2 cache effect that's visible in these in-memory tests:

 select-medsize-patched            | 00:00:01.332975
 select-medsize-patched            | 00:00:01.33014
 select-medsize-patched            | 00:00:01.332392
 select-medsize-patched            | 00:00:01.333498
 select-medsize-patched            | 00:00:01.332692
 select-medsize-unpatched          | 00:00:02.678069
 select-medsize-unpatched          | 00:00:02.666103
 select-medsize-unpatched          | 00:00:02.673494
 select-medsize-unpatched          | 00:00:02.669645
 select-medsize-unpatched          | 00:00:02.666278

is also visible on larger scans that don't fit in cache with bigger I/O hardware, and this patch would increase the max. I/O throughput that we can handle on such hardware. I don't have such hardware available, I hope someone else will try that.

In addition to that, this should reduce the cache-spoiling effect of big scans and COPY. That's harder to quantify, I've been planning to run a TPC-C test with a large SELECT COUNT(*) running in the background to measure that, but me and the server has been busy with those other tests.

In any case, I do want this for VACUUMs to fix the "WAL flush for every dirty page" problem. Maybe we should indeed drop the other aspects of the patch and move on, I'm getting tired of this as well.

  Heikki Linnakangas

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