svn commit: r253002 - head

[Garrett Cooper]

On Sun, Jul 7, 2013 at 7:25 PM, Garrett Cooper <yaneurabeya at gmail.com> wrote:
> On Jul 7, 2013, at 2:15 PM, Alfred Perlstein <alfred at freebsd.org> wrote:
>
>> On 7/7/13 2:01 PM, Garrett Cooper wrote:
>>> Why the magic number 12?
>>
>> Numbers higher seem to result in worse performance as reported by some members of my team.
>
> The suggestion is good in spirit, but this doesn't justify the reasoning for this recommendation for all cases.
>
> Please revert this change and add a doc page or notes to the dev handbook discussing what the empirical process and results were for determining this value so people can come up with their own values that work best with their hardware and software config. This recommendation is prone to bitrot like some of the recommendations in tuning(7).
>
> Misinformation is sometimes more harmful than no information.

I spoke with Alfred over the phone and did some more careful thought
about this and I'm rescinding this request.

Alfred did a good job at documenting how JFLAG works (it was
previously undocumented). My concern over -j12 was performance
related, and after giving things more careful thought it actually
makes sense why -j12 was chosen because Westmere and newer processors
have issues with NUMA and cache locality between multiple processor
packages as we've seen non-empirically and empirically at Isilon with
FreeBSD 7 and 10 (it's a known issue that jeffr@ and jhb@ are aware
of).

I'll come up with a concise patch that does what Alfred was trying to
achieve and have Alfred review it.

Thanks (and thank you Alfred for the contribution!!!)!
-Garrett