Re: checkpointer continuous flushing - V18

On 2016-02-22 20:44:35 +0100, Fabien COELHO wrote:
>
> >>Random updates on 16 tables which total to 1.1GB of data, so this is in
> >>buffer, no significant "read" traffic.
> >>
> >>(1) with 16 tablespaces (1 per table) on 1 disk : 680.0 tps
> >> per second avg, stddev [ min q1 median d3 max ] <=300tps
> >> 679.6 ± 750.4 [0.0, 317.0, 371.0, 438.5, 2724.0] 19.5%
> >>
> >>(2) with 1 tablespace on 1 disk : 956.0 tps
> >> per second avg, stddev [ min q1 median d3 max ] <=300tps
> >> 956.2 ± 796.5 [3.0, 488.0, 583.0, 742.0, 2774.0] 2.1%
> >
> >Interesting. That doesn't reflect my own tests, even on rotating media,
> >at all. I wonder if it's related to:
> >https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=23d0127096cb91cb6d354bdc71bd88a7bae3a1d5
> >
> >If you use your 12.04 kernel, that'd not be fixed. Which might be a
> >reason to do it as you suggest.
> >
> >Could you share the exact details of that workload?
>
> See attached scripts (sh to create the 16 tables in the default or 16 table
> spaces, small sql bench script, stat computation script).
>
> The per-second stats were computed with:
>
> grep progress: pgbench.out | cut -d' ' -f4 | avg.py --length=1000 --limit=300
>
> Host is 8 cpu 16 GB, 2 HDD in RAID 1.

Well, that's not a particularly meaningful workload. You increased the
number of flushed to the same number of disks considerably. For a
meaningful comparison you'd have to compare using one writeback context
for N tablespaces on N separate disks/raids, and using N writeback
contexts for the same.

Andres