On 15.04.2013 21:42, Pawel Jakub Dawidek wrote:
On Sat, Apr 13, 2013 at 12:59:49PM +0300, Alexander Motin wrote:
Hi.
It is long known that collecting disk and GEOM statistics may cause
significant processing overhead under high IOPS. On my recent high-IOPS
benchmarks performance difference was reaching three times! Last time
situation improved a lot by more active use of TSC, but there are still
many systems where TSCs are not synchronized. I propose to switch that
statistics from using binuptime() to getbinuptime() to solve the problem
globally.
From one side getbinuptime() resolution is limited by 1ms, but since
time is usually averaged over the many I/Os, additional sub-millisecond
precision will come from sampling. Since most of tools now show request
processing times up to 0.1ms, that precision should be sufficient. I
believe real disk performance is more important that n-th digit in some
statistics.
The following patch does the change and makes disk performance
irrelevant to the timecounter performance:
http://people.freebsd.org/~mav/devstat_time.patch
Are there any objections against it?
No objections here, but I wonder if you were able to compare the results
somehow before and after the change so we have some hard numbers to show
that we don't lose much by applying the change.
I haven't tested it statistically, but I haven't noticed any visual
difference in gstat output with its 0.1ms displayed resolution.
On a mostly unrelated note when two threads (T0 and T1) call get*time()
on two different cores, but T0 does that a bit earlier is it possible
that T0 can get later time than T1?
There is no any locking or memory barriers in get*time(), so CPU is free
to do some prefetching and out-of-order execution, slightly blurring "a
bit earlier" concept. Except such close cases I believe it should not
happen.
--
Alexander Motin
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