Re: git bisect on multiple cores

From: Matthieu Moy <matthieu.moy@imag.fr>
Date: Apr 9, 2008, 08:07 UTC
Message-ID: <vpq63ure9j6.fsf@bauges.imag.fr>
In-Reply-To: <20080409071750.GA23211@diana.vm.bytemark.co.uk>

Karl Hasselström <kha@treskal.com> writes:

Show 13 quoted lines

> On 2008-04-08 12:58:47 +0200, A B wrote:
>
>> git bisect is really cool. I use it for the first time. Just a
>> thought, if you have a multiple core computer, can't you make git
>> build new versions in the background while testing the previuos
>> version? Alright, if you build 2 versions, one of them will never be
>> tested, but you will perhaps save some time by letting it build in
>> the background?
>
> 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.

And as said before, you can also try trisection or N-section. But no, I won't claim this is simple to implement ;-).

-- 
Matthieu