[GHC] #910: --make should have a -j flag for parallel building

[GHC]

#910: --make should have a -j flag for parallel building
-------------------------------------+------------------------------------
        Reporter:  igloo             |            Owner:
            Type:  feature request   |           Status:  patch
        Priority:  normal            |        Milestone:  _|_
       Component:  Compiler          |          Version:  6.4.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:  N/A               |       Blocked By:  8184
        Blocking:                    |  Related Tickets:
-------------------------------------+------------------------------------

Comment (by rrnewton):

 Ok, I'm trying to get a decent set of libraries installed to test this
 well.  The very first thing I cabal installed ('text') did get a small
 speedup.

 However, I'm also seeing some excessive system time.  This may have
 nothing to do with the parallel make approach and just be a function of
 the new IO manager.  In fact, if I understand the design, worker threads
 in this model should either be running or blocked on MVars.  (That's good
 for avoiding wasted user time as well, unlike work-stealing which burns
 cycles looking for work.)

 I'm running on a 32-core Intel Westmere machine, using this command to
 install `text` version 0.11.3.1:

 {{{
 time cabal install text --ghc-options="-j24" --reinstall
 }}}

 Notice that in this simple test I am relying on the `setNumCapabilities`
 behavior.  Here are the times:

 {{{
  * 1 thread:   real 1m20.028s user 1m17.921s sys 0m1.768s
  * 2 threads:  real 1m7.417s user 1m22.818s sys 0m14.891s
  * 4 threads:  real 0m59.528s user 1m29.110s sys 0m37.981s
  * 8 threads:  real 0m57.219s user 1m54.461s sys 1m31.703s
  * 16 threads: real 1m6.225s user 4m46.976s sys 3m32.661s
  * 24 threads: real 1m16.501s user 9m53.254s sys 6m3.375s
  * 31 threads: real 1m27.445s user 17m0.314s sys 8m0.175s
 }}}

 Well, it's nice that final sequential time is not much worse than the one-
 threaded time!

 Finally, here is the fingerprint:

 {{{
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 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/910#comment:39>
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