On Sat, Aug 22, 2020 at 01:16:07AM +0000, brian m. carlson wrote: > > + if (nr_threads < 4) > > + ; /* too few cores to consider capping */ > > + else if (nr_threads < 6) > > + nr_threads = 3; /* historic cap */ > > + else if (nr_threads < 40) > > + nr_threads /= 2; > > I was going to ask if we could make the halving conditional on x86_64, > but it turns out POWER and UltraSPARC also have SMT, so that doesn't > make sense. I expect that most users who have more than 6 "cores" are > going to be on one of those systems or possibly ARM64, and since the > performance penalty of using half as many cores isn't that significant, > I'm not sure it's worth worrying about further. This will be an > improvement regardless. > > Which is just a long way of saying, this patch seems fine to me. OK, good. :) I agree there may be room for more improvement on those systems. But lacking access to any, my goal was to make things better on systems I _could_ test on, and not make things worse on other systems. So I'd be very happy if people on other platforms (especially non-intel ones) wanted to run: cd t/perf GIT_PERF_EXTRA=1 \ GIT_PERF_LARGE_REPO=/path/to/clone/of/linux.git \ ./p5302-pack-index.sh and report the results. I do have a slightly-old AMD machine with 4 cores (an A8-7600). Here's what it says: 5302.3: index-pack 0 threads 447.67(436.62+6.57) 5302.4: index-pack 1 threads 450.80(441.26+7.20) 5302.5: index-pack 2 threads 265.62(459.56+7.30) 5302.6: index-pack 4 threads 177.06(477.56+8.22) 5302.7: index-pack default number of threads 202.60(473.15+7.61) So it does get better with 4 threads (but we continue to cap it at 3). I wonder whether we should just do: diff --git a/builtin/index-pack.c b/builtin/index-pack.c index 9721bf1ffe..d7453d0c09 100644 --- a/builtin/index-pack.c +++ b/builtin/index-pack.c @@ -1809,7 +1809,7 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix) if (nr_threads < 4) ; /* too few cores to consider capping */ else if (nr_threads < 6) - nr_threads = 3; /* historic cap */ + nr_threads = nr_threads; else if (nr_threads < 40) nr_threads /= 2; else That does probably make things slightly worse for a 6-core hyperthreaded Intel machine. And it doesn't help an actual 8-core AMD machine at all. -Peff
