On Fri, Nov 03 2006, Brent Baccala wrote:
> On Fri, 3 Nov 2006, Jens Axboe wrote:
>
> >Try to time it (visual output of the app is not very telling, and it's
> >buffered) and then apply some profiling.
>
> OK, a little more info. I added gettimeofday() calls after each call
> to io_submit(), put the timevals in an array, and after everything was
> done computed the difference between each timeval and the program start
> time, as well as the deltas. I got this:
>
> 0: 0.080s
> 1: 0.086s 0.006s
> 2: 0.102s 0.016s
> 3: 0.111s 0.008s
> 4: 0.118s 0.007s
> 5: 0.134s 0.015s
> 6: 0.141s 0.006s
> 7: 0.148s 0.006s
> 8: 0.158s 0.009s
> 9: 0.164s 0.006s
> ...
> 96: 1.036s 0.007s
> 97: 1.044s 0.007s
> 98: 1.147s 0.102s
> 99: 1.155s 0.008s
>
> 98 appears to be an aberration. Perhaps three of the times on an
> average run are around a tenth of a second; all of the others are
> pretty steady at 7 or 8 microseconds. So, it's basically linear in
> its time consumption.
>
> Does 7 microseconds seem a bit excessive for an io_submit (and a
> gettimeofday)?
I guess you mean miliseconds, not microseconds. 7 miliseconds seems way
too long. I repeated your test here, and the 100 submits take 97000
microseconds here - or 97 miliseconds. So that's a little less than 1
msec per io_submit. Still pretty big. You can experiment with oprofile
to profile where the kernel spends its time in that period.
--
Jens Axboe
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