On Wed, Feb 1, 2012 at 5:47 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Jan 25, 2012 at 8:49 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Tue, Jan 24, 2012 at 4:28 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>>> I think we should be working to commit XLogInsert and then Group
>>> Commit, then come back to the discussion.
>>
>> I definitely agree that those two have way more promise than anything
>> else on the table. However, now that I understand how badly we're
>> getting screwed by checkpoints, I'm curious to do some more
>> investigation of what's going on there. I can't help thinking that in
>> these particular cases the full page writes may be a bigger issue than
>> the checkpoint itself. If that turns out to be the case it's not
>> likely to be something we're able to address for 9.2, but I'd like to
>> at least characterize it.
>
> I did another run with full_page_writes=off:
>
> http://wiki.postgresql.org/wiki/File:Tps-master-nofpw.png
>
> This is run with the master branch as of commit
> 4f42b546fd87a80be30c53a0f2c897acb826ad52, same as the previous tests,
> so that the results are comparable.
>
> The graph is pretty dramatically different from the earlier one:
>
> http://wiki.postgresql.org/wiki/File:Tps-master.png
>
> So are the TPS numbers:
>
> full page writes on: tps = 2200.848350 (including connections establishing)
> full page writes off: tps = 9494.757916 (including connections establishing)
>
> It seems pretty clear that, even with full_page_writes=off, the
> checkpoint is affecting things negatively: the first 700 seconds or so
> show much better and more consistent performance than the remaining
> portion of the test. I'm not sure why that is, but I'm guessing there
> was a checkpoint around that time. But the effect is much worse with
> full_page_writes=on: the distinctive parabolic shape of those graphs
> is apparently caused by the gradually decreasing frequency of full
> page writes as the number of transactions processed since the last
> checkpoint grows.
Sounds like time to test the checkpoint smoothing patch.
--
Simon Riggs http://www.2ndQuadrant.com/
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