GhcMake: limit Capability count to CPU count in parallel mode
authorSergei Trofimovich <slyfox@gentoo.org>
Tue, 30 Aug 2016 11:10:47 +0000 (12:10 +0100)
committerSergei Trofimovich <siarheit@google.com>
Tue, 30 Aug 2016 11:11:06 +0000 (12:11 +0100)
commit9d175605e52fd0d85f2548896358d96ee441c7e4
tree362e73fbf3af21ed43e997f02d5fc53a02d4dda6
parent21c2ebf2504125611eec0d24ab3ed60cd9e475be
GhcMake: limit Capability count to CPU count in parallel mode

In Trac #9221 one of problems using high --jobs=<N>
is amount of mutator (or GC) threads we crate.

We use userspace spinning-and-yielding (see ACQUIRE_SPIN_LOCK)
to acess work stealing queues. In case of
N-worker-threads > N-CPUs fraction of time when
thread holding spin lock gets descheduled by kernel
increases. That causes other threads to waste CPU time
before giving up CPU.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>
Test Plan:
ghc --make -j8 and -j80 have comparable sys time
on a 8-core system.

Reviewers: austin, gintas, bgamari, simonmar

Reviewed By: bgamari, simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2482

GHC Trac Issues: #9221
compiler/main/GhcMake.hs