Re: PCTFree Results

Josh Berkus <josh(at)agliodbs(dot)com> writes:
> ... The good news is that it appears that stuff which has been
> done since July has lessened the penalty for checkpoints somewhat; while
> the maximum response time is still better on the full_page_writes=off
> systems, the average throughput is no longer substantially different.
> Either that, or full_page_writes=off isn't working properly anymore.

I dug through the CVS logs since 5-July (when full_page_writes was
added, so I suppose that's before your unspecified "July" test).
AFAICS the only changes that might possibly affect xlog/checkpoint
performance were these:

Use O_DIRECT if available when using O_SYNC for wal_sync_method.

Also, write multiple WAL buffers out in one write() operation.

Was your test set up so that it would have used O_DIRECT?

With respect to the original point, I'm pretty nervous about either
accepting or rejecting a performance-oriented patch on the strength
of a single test case. This report certainly doesn't favor the PCTFREE
patch, but it probably shouldn't kill it either. Anyone want to try it
on some other test cases?

regards, tom lane