On Wed, Mar 30, 2016 at 7:23 AM, Peter Geoghegan <p...@heroku.com> wrote:
> Anyway, what I liked about Greg's approach to finding regressions at
> the low end was that when testing, he used the cheapest possible VM
> available on Google's cloud platform. When testing the low end, he had
> low end hardware to go with the low end work_mem settings. This gave
> the patch the benefit of using quicksort to make good use of what I
> assume is a far smaller L2 cache; certainly nothing like 6MB or 12MB.
> I think Greg might have used a home server to test my patch in [1],
> actually, but I understand that it too was suitably low-end.

I'm sorry I was intending to run those benchmarks again this past week
but haven't gotten around to it. But my plan was to run them on a good
server I borrowed, an i7 with 8MB cache. I can still go ahead with
that but I can also try running it on the home server again too if you
want (and AMD N36L with 1MB cache).

But even for the smaller machines I don't think we should really be
caring about regressions in the 4-8MB work_mem range. Earlier in the
fuzzer work I was surprised to find out it can take tens of megabytes
to compile a single regular expression (iirc it was about 30MB for a
64-bit machine) before you get errors. It seems surprising to me that
a single operator would consume more memory than an ORDER BY clause. I
was leaning towards suggesting we just bump up the default work_mem to
8MB or 16MB.


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