Nguyễn Thái Ngọc Duy <pclouds@xxxxxxxxx> writes:
> This puts delta resolving on each base on a separate thread, one base
> cache per thread. Per-thread data is grouped in struct thread_local.
> When running with nr_threads == 1, no pthreads calls are made. The
> system essentially runs in non-thread mode.
As discussed when we took the git-grep measurements, it may be
interesting to have a way to run 1 thread. Can you put in such an
option?
> An experiment on a Xeon 24 core machine with linux-2.6.git shows that
> performance does not increase proportional to the number of cores. So
> by default, we use maximum 3 cores. Some numbers with --threads from 1
> to 16:
>
> 1..4
> real 1m16.310s 0m48.183s 0m37.866s 0m32.834s
> user 1m13.773s 1m15.537s 1m15.781s 1m16.233s
> sys 0m2.480s 0m3.936s 0m4.448s 0m4.852s
>
> 5..8
> real 0m33.170s 0m30.369s 0m28.406s 0m26.968s
> user 1m31.474s 1m30.322s 1m29.562s 1m28.694s
> sys 0m6.096s 0m6.268s 0m6.684s 0m7.172s
Interesting. Is this a real 24-core machine or 12*2 hyperthreaded?
Does it use Turbo Boost and how far (how fast and on how many cores
simultaneously) does that go?
I'm asking because if Turbo Boost starts to wear off around 4 cores,
like these measurements suggest, then it may not be beneficial to spawn
threads on 2*2HT CPUs (found in many laptops) where Turbo Boost only
really works if you only use a single core.
Oh, and could you write a perf test for this? :-)
--
Thomas Rast
trast@{inf,student}.ethz.ch
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