[GHC] #9221: (super!) linear slowdown of parallel builds on 40 core machine

[GHC]

#9221: (super!) linear slowdown of parallel builds on 40 core machine
-------------------------------------+-------------------------------------
              Reporter:  carter      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  high        |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.8.2
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  Compile-    |       Blocked By:
  time performance bug               |  Related Tickets:  #910
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by carter):

 Yeah, I definitely found that making the nursery larger helped A LOT, i'd
 done `-A50m` or `-A100m` on a few occasions, though I must admit I didn't
 think to do that for running GHC itself!

 Good point about hyperthreads, that would match my experience of only
 seeing good perf up to about 15-20ish for -N.
 This does raise the problem that the default -N flag should probably be
 set to the number of physical cores rather than hyper threaded cores,
 would that be a reasonable patch for me to write for GHC? (though of
 course theres the matter of correctly detecting that info in a portable
 way! )

 I unfortunately dont have access anymore to that machine (or presently any
 SSH access to a mega core sized box) so I can't repro it to dig in deeper,
 I was using some cli tool who's name escapes me right now

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9221#comment:13>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler