Re: [PERFORM] 8.3beta1 testing on Solaris - Mailing list pgsql-hackers

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Jignesh K. Shah wrote:
>
> I  changed  CLOG Buffers to 16
>
> Running the test again:
> # ./read.d
> dtrace: script './read.d' matched 2 probes
> CPU     ID                    FUNCTION:NAME
>   0   1027                       :tick-5sec
>
>   /export/home0/igen/pgdata/pg_clog/0024
> -2753028219296                1
>   /export/home0/igen/pgdata/pg_clog/0025
> -2753028211104                1
> # ./read.d
> dtrace: script './read.d' matched 2 probes
> CPU     ID                    FUNCTION:NAME
>   1   1027                       :tick-5sec
>
> # ./read.d
> dtrace: script './read.d' matched 2 probes
> CPU     ID                    FUNCTION:NAME
>   1   1027                       :tick-5sec
>
> # ./read.d
> dtrace: script './read.d' matched 2 probes
> CPU     ID                    FUNCTION:NAME
>   0   1027                       :tick-5sec
>
>   /export/home0/igen/pgdata/pg_clog/0025
> -2753028194720                1
>
>
> So Tom seems to be correct that it is a case of CLOG Buffer thrashing.
> But since I saw the same problem with two different workloads, I think
> people hitting this problem is pretty high.
>
> Also I am bit surprised that CLogControlFile did not show up as being
> hot.. Maybe because not much writes are going on .. Or maybe since I did
> not trace all 500 users to see their hot lock status..
>
>
> Dmitri has another workload to test, I might try that out later on to
> see if it causes similar impact or not.
>
> Of course I havent seen my throughput go up yet since I am already CPU
> bound... But this is good since the number of IOPS to the disk are
> reduced (and hence system calls).
>
>
> If I take this as my baseline number.. I can then proceed to hunt other
> bottlenecks????
>
>
> Whats the view of the community?
>
> Hunt down CPU utilizations or Lock waits next?
>
> Your votes are crucial on where I put my focus.
>
> Another thing Josh B told me to check out was the wal_writer_delay setting:
>
> I have done two settings with almost equal performance (with the CLOG 16
> setting) .. One with 100ms and other default at 200ms.. Based on the
> runs it seemed that the 100ms was slightly better than the default ..
> (Plus the risk of loosing data is reduced from 600ms to 300ms)
>
> Thanks.
>
> Regards,
> Jignesh
>
>
>
>
> Tom Lane wrote:
> > "Jignesh K. Shah" <J.K.Shah@Sun.COM> writes:
> >
> >> So the ratio of reads vs writes to clog files is pretty huge..
> >>
> >
> > It looks to me that the issue is simply one of not having quite enough
> > CLOG buffers.  Your first run shows 8 different pages being fetched and
> > the second shows 10.  Bearing in mind that we "pin" the latest CLOG page
> > into buffers, there are only NUM_CLOG_BUFFERS-1 buffers available for
> > older pages, so what we've got here is thrashing for the available
> > slots.
> >
> > Try increasing NUM_CLOG_BUFFERS to 16 and see how it affects this test.
> >
> >             regards, tom lane
> >
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