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?
--
Alexander Motin
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