On 2008-04-09 10:07:25 +0200, Matthieu Moy wrote:
> Karl Hasselström <kha@xxxxxxxxxxx> writes:
>
> > Adding parallelism to a binary search scales very badly -- I'd say
> > about logarithmically, but I haven't thought hard about it. If
> > it's possible to use the extra cores to speed up the build+test
> > cycle, that's vastly preferable.
>
> Probably logarithmically with the number of cores. But for
> reasonable machines, this number is relatively low, so the log is
> not so costly. For a binary search, using just 2 cores, you can try
> the next in the list in case of a "git bisect good" for example, and
> if the hypothesis is true, you've just gained a factor 2 (assuming
> it happens 50% of times, that should be a 50% speedup). Similarly,
> you should get a factor 2 with 3 cores.
Yeah. But to get a factor 3, you need 7 cores; and for 4, you need 15.
It goes downhill from there. If your build+test cycle is
parallelizable at all, I don't think you'll find those numbers hard to
beat.
(There's also the fact that testing several revisions at once assumes
that the whole build+test cycle is automated, or at least most of it.
Otherwise you need more people as well as more cores.)
--
Karl Hasselström, kha@xxxxxxxxxxx
www.treskal.com/kalle
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