On Wed, Feb 1, 2012 at 5:47 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Wed, Jan 25, 2012 at 8:49 AM, Robert Haas <robertmh...@gmail.com> wrote: >> On Tue, Jan 24, 2012 at 4:28 PM, Simon Riggs <si...@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/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers