svn commit: r247319 - in projects/calloutng/sys: kern sys

[Attilio Rao]

On Tue, Feb 26, 2013 at 4:46 PM, Alexander Motin <mav at freebsd.org> wrote:
> On 26.02.2013 17:28, Attilio Rao wrote:
>> On Tue, Feb 26, 2013 at 4:25 PM, Alexander Motin <mav at freebsd.org> wrote:
>>> Author: mav
>>> Date: Tue Feb 26 15:25:43 2013
>>> New Revision: 247319
>>> URL: http://svnweb.freebsd.org/changeset/base/247319
>>>
>>> Log:
>>>   Optimize callout_process() to use less variables and less conditions to
>>>   implement the same logic.  Now it fits better into CPU registers, and
>>>   according to PMC significntly reduces number of resource stalls, reducing
>>>   consumed by it CPU time during usleep(1) benchmark by 30%.
>>
>> Is that all improved i-cache capacity and improved dynamic branch
>> prediction (hwpmc has counters for both FWIW)?
>
> I-cache capacity I think is not significant there as the loop is quite
> small. I believe it was branch misprediction, complicated by additional
> latency of memory accesses. I haven't analyzed cause deeper, as PMC man
> pages are not the most informative and easiest reading.

Well, I-cache is really very small, so I think you may get some
improvement also for the function you were trying to optimize.
You can get all the counter description by doing: pmccontrol -L
>From there you may find some hwpmc counter showing i-cache and dynamic
branch prediction misses statistics.

Attilio


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