Re: Remove the comment on the countereffectiveness of large shared_buffers on Windows

On Tue, Nov 8, 2016 at 5:03 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:

> On Tue, Nov 8, 2016 at 5:12 AM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> > On Mon, Nov 7, 2016 at 5:55 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
> >>
> >> On Tue, Sep 20, 2016 at 8:15 AM, Tsunakawa, Takayuki
> >> <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com> wrote:
> >> > I ran read-only and read-write modes of pgbench, and could not see any
> >> > apparent decrease in performance when I increased shared_buffers. The
> >> > scaling factor is 200, where the database size is roughly 3GB. I ran
> the
> >> > benchmark on my Windows 10 PC with 6 CPU cores and 16GB of RAM. The
> >> > database and WAL is stored on the same HDD.
> >> >
> >> > <<Test batch file>>
> >> > @echo off
> >> > for %%s in (256MB 512MB 1GB 2GB 4GB) do (
> >> > pg_ctl -w -o "-c shared_buffers=%%s" start
> >> > pgbench -c18 -j6 -T60 -S bench >> g:\b.txt 2>&1
> >> > pg_ctl -t 3600 stop
> >> > )
> >> >
> >> > <<Select-only (with -S)>>
> >> > shared_buffers tps
> >> > 256MB 63056
> >> > 512MB 63918
> >> > 1GB 65520
> >> > 2GB 66840
> >> > 4GB 68270
> >> >
> >> > <<Read-write (without -S)>>
> >> > shared_buffers tps
> >> > 256MB 1138
> >> > 512MB 1187
> >> > 1GB 1571
> >> > 2GB 1650
> >> > 4GB 1598
> >> >
> >>
> >> Isn't it somewhat strange that writes are showing big win whereas
> >> reads doesn't show much win?
> >
> >
> > I don't find that unusual, and have seen the same thing on Linux.
> >
> > With small shared_buffers, you are constantly throwing dirty buffers at
> the
> > kernel in no particular order, and the kernel does a poor job of
> predicting
> > when the same buffer will be dirtied repeatedly and only needs the final
> > version of the data actually written to disk.
> >
>
> Okay and I think partially it might be because we don't have writeback
> optimization (done in 9.6) for Windows. However, still the broader
> question stands that whether above data is sufficient to say that we
> can recommend the settings of shared_buffers on Windows similar to
> Linux?
>
>
Based on this optimization we might want to keep the text that says large
shared buffers on Windows aren't as effective perhaps, and just remove the
sentence that explicitly says don't go over 512MB?

--
Magnus Hagander
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