Re: BitmapHeapScan streaming read user and prelim refactoring

On 3/18/24 16:55, Tomas Vondra wrote:
>
> ...
> 
> OK, I've restarted the tests for only 0012 and 0014 patches, and I'll
> wait for these to complete - I don't want to be looking for patterns
> until we have enough data to smooth this out.
> 
>

I now have results for 1M and 10M runs on the two builds (0012 and
0014), attached is a chart for relative performance plotting

  (0014 timing) / (0012 timing)

for "optimal' runs that would pick bitmapscan on their own. There's
nothing special about the config - I reduced the random_page_cost to
1.5-2.0 to reflect both machines have flash storage, etc.

Overall, the chart is pretty consistent with what I shared on Sunday.
Most of the results are fine (0014 is close to 0012 or faster), but
there's a bunch of cases that are much slower. Interestingly enough,
almost all of them are on the i5 machine, almost none of the xeon. My
guess is this is about the SSD type (SATA vs. NVMe).

Attached if table of ~50 worst regressions (by the metric above), and
it's interesting the worst regressions are with eic=0 and eic=1.

I decided to look at the first case (eic=0), and the timings are quite
stable - there are three runs for each build, with timings close to the
average (see below the table).

Attached is a script that reproduces this on both machines, but the
difference is much more significant on i5 (~5x) compared to xeon (~2x).

I haven't investigated what exactly is happening and why, hopefully the
script will allow you to reproduce this independently. I plan to take a
look, but I don't know when I'll have time for this.

FWIW if the script does not reproduce this on your machines, I might be
able to give you access to the i5 machine. Let me know.


regards

-- 
Tomas Vondra
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