Re: Perf Benchmarking and regression.

On 2016-05-13 14:43:15 -0400, Robert Haas wrote:
> On Fri, May 13, 2016 at 1:43 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > I just want to emphasize what we're discussing here is a bit of an
> > extreme setup. A workload that's bigger than shared buffers, but smaller
> > than the OS's cache size; with a noticeable likelihood of rewriting
> > individual OS page cache pages within 30s.
>
> You're just describing pgbench with a scale factor too large to fit in
> shared_buffers.

Well, that *and* a scale factor smaller than 20% of the memory
available, *and* a scale factor small enough that make re-dirtying of
already written out pages likely.

> I think it's unfair to paint that as some kind of niche use case.

I'm not saying we don't need to do something about it. Just that it's a
hard tradeoff to make. The massive performance / latency we've observed
originate from the kernel caching too much dirty IO. The fix is making
is cache fewer dirty pages. But there's workloads where the kernel's
buffer cache works as an extension of our page cache.