Re: postgresql.conf recommendations

I originally got started down that trail because running perf top while
having some of the slow query issues showed compaction_alloc at the top of
the list. That function is the THP page compaction which lead me to some
pages like:
http://www.olivierdoucet.info/blog/2012/05/19/debugging-a-mysql-stall/
http://structureddata.org/2012/06/18/linux-6-transparent-huge-pages-and-hadoop-workloads/
https://gist.github.com/fgbreel/4454559
Only the last is around pg, but I think they all may still be applicable.

The kernel docs in Documentation/vm/transhuge.txt have an explanation of
the metrics

We hadn't been having the issue that much until a few weeks ago, when we
started using the rest of our free memory for page cache.. my thoughts were
if we have no more memory that's totally free, it might be doing compaction
more. That lead me to find how often compaction is happening, but like I
said I don't know how to tell how *long* it's happening - someone who knows
systemtap better than I might be able to help with the collection of that
info assuming the right systemtap events are there.

One thing to keep in mind is the page you linked to (
http://www.pythian.com/blog/performance-tuning-hugepages-in-linux/) talks
mostly about *regular* large pages, which are related but different than
THP.

I have yet to be able to prove THP's involvement one way or the other, but
we are going to try some things on a test box to zero in on it.