Re: Perf Benchmarking and regression.

Hello,

> Please find the results for the following 3 scenarios with unpatched master:
>
> 1. Default settings for *_flush_after : TPS = *10677.662356*
> 2. backend_flush_after=0, rest defaults : TPS = *18452.655936*
> 3. backend_flush_after=0, bgwriter_flush_after=0,
> wal_writer_flush_after=0, checkpoint_flush_after=0 : TPS = *18614.479962*

Thanks for these runs.

These raw tps suggest that {backend,bgwriter}_flush_after should better be
zero for this kind of load.Whether it should be the default is unclear
yet, because as Andres pointed out this is one kind of load.

Note: these options have been added to smooth ios over time and to help
avoid "io panics" on sync, especially with HDDs without a large BBU cache
in front. The real benefit is that the performance are much more constant
over time, and pg is much more responsive.

If you do other runs, it would be nice to report some stats about tps
variability (eg latency & latency stddev which should be in the report).
For experiments I did I used to log "-P 1" output (tps every second) and
to compute stats on these tps (avg, stddev, min, q1, median, q3, max, pc
of time with tps below a low threshold...), which provides some indication
of the overall tps distribution.

--
Fabien