Date: Sun, 7 Jul 2013 23:23:31 -0700 From: Garrett Cooper <yaneurabeya@gmail.com> To: Alfred Perlstein <alfred@freebsd.org> Cc: "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "src-committers@freebsd.org" <src-committers@freebsd.org> Subject: Re: svn commit: r253002 - head Message-ID: <CAGHfRMDmRXR1jTF_CO4T8NXOPW5Soxjrq1y_eb41mKZG2yQiiw@mail.gmail.com> In-Reply-To: <D3E9FF9F-E53A-40DE-8DFF-C4A4F05566D4@gmail.com> References: <201307072039.r67KdCdR028908@svn.freebsd.org> <9D4C7540-A3B0-45E5-8219-6A455D41DF70@gmail.com> <51D9DA55.2090808@freebsd.org> <D3E9FF9F-E53A-40DE-8DFF-C4A4F05566D4@gmail.com>
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On Sun, Jul 7, 2013 at 7:25 PM, Garrett Cooper <yaneurabeya@gmail.com> wrot= e: > On Jul 7, 2013, at 2:15 PM, Alfred Perlstein <alfred@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 m= embers 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 thi= s value so people can come up with their own values that work best with the= ir hardware and software config. This recommendation is prone to bitrot lik= e 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
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