devstat overhead VS precision

[Pawel Jakub Dawidek]

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.

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?

-- 
Pawel Jakub Dawidek                       http://www.wheelsystems.com
FreeBSD committer                         http://www.FreeBSD.org
Am I Evil? Yes, I Am!                     http://mobter.com
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