Greg Smith wrote:
On Mon, 5 May 2008, Craig James wrote:
pgbench -i -s 20 -U test
That's way too low to expect you'll see a difference in I/O schedulers.
A scale of 20 is giving you a 320MB database, you can fit the whole
thing in RAM and almost all of it on your controller cache. What's
there to schedule? You're just moving between buffers that are
generally large enough to hold most of what they need.
Test repeated with:
autovacuum enabled
database destroyed and recreated between runs
pgbench -i -s 600 ...
pgbench -c 10 -t 50000 -n ...
I/O Sched AVG Test1 Test2
--------- ----- ----- -----
cfq 705 695 715
noop 758 769 747
deadline 741 705 775
anticipatory 494 477 511
I only did two runs of each, which took about 24 minutes. Like the first round of tests, the
"noise" in the measurements (about 10%) exceeds the difference between
scheduler-algorithm performance, except that "anticipatory" seems to be measurably slower.
So it still looks like cfq, noop and deadline are more or less equivalent when
used with a battery-backed RAID.
Craig
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