On Fri, Sep 13, 2013 at 10:52 AM, Merlin Moncure <mmonc...@gmail.com> wrote:
> On Thu, Sep 12, 2013 at 3:06 PM, David Whittaker <d...@iradix.com> wrote: > > Hi All, > > > > We lowered shared_buffers to 8G and increased effective_cache_size > > accordingly. So far, we haven't seen any issues since the adjustment. > The > > issues have come and gone in the past, so I'm not convinced it won't > crop up > > again, but I think the best course is to wait a week or so and see how > > things work out before we make any other changes. > > > > Thank you all for your help, and if the problem does reoccur, we'll look > > into the other options suggested, like using a patched postmaster and > > compiling for perf -g. > > > > Thanks again, I really appreciate the feedback from everyone. > > Interesting -- please respond with a follow up if/when you feel > satisfied the problem has gone away. Andres was right; I initially > mis-diagnosed the problem (there is another issue I'm chasing that has > a similar performance presentation but originates from a different > area of the code). > > That said, if reducing shared_buffers made *your* problem go away as > well, then this more evidence that we have an underlying contention > mechanic that is somehow influenced by the setting. Speaking frankly, > under certain workloads we seem to have contention issues in the > general area of the buffer system. I'm thinking (guessing) that the > problems is usage_count is getting incremented faster than the buffers > are getting cleared out which is then causing the sweeper to spend > more and more time examining hotly contended buffers. This may make > no sense in the context of your issue; I haven't looked at the code > yet. Also, I've been unable to cause this to happen in simulated > testing. But I'm suspicious (and dollars to doughnuts '0x347ba9' is > spinlock related). > > Anyways, thanks for the report and (hopefully) the follow up. > > merlin > You guys have taken the time to help me through this, following up is the least I can do. So far we're still looking good.
