[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:
       Component:  Compiler                      |          Version:  7.8.2
      Resolution:                                |         Keywords:
Operating System:  Unknown/Multiple              |     Architecture:
 Type of failure:  Compile-time performance bug  |  Unknown/Multiple
       Test Case:                                |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:
                                                 |  Related Tickets:
-------------------------------------------------+-------------------------

Comment (by slyfox):

 I've noticed it as well. For superficial tests -jN>4 does not make much
 sense even on 8-core box.

 Attached a cabal build test (takes ~50 seconds to build). Use as:
 {{{
 $ cd $ghc-source
 ./test-ghc-cabal.sh >build-log
 }}}

 Results for my machine:
 {{{
 RUN jobs: 1

 real    0m51.745s
 user    0m49.825s
 sys     0m1.553s
 RUN jobs: 2

 real    0m34.254s
 user    0m57.075s
 sys     0m6.278s
 RUN jobs: 3

 real    0m31.670s
 user    1m7.058s
 sys     0m10.970s
 RUN jobs: 4

 real    0m32.008s
 user    1m10.548s
 sys     0m18.194s
 RUN jobs: 5

 real    0m32.329s
 user    1m15.384s
 sys     0m27.939s
 RUN jobs: 6

 real    0m33.993s
 user    1m25.190s
 sys     0m41.473s
 RUN jobs: 7

 real    0m35.410s
 user    1m32.354s
 sys     0m51.201s
 RUN jobs: 8

 real    0m36.111s
 user    1m42.945s
 sys     1m1.740s
 RUN jobs: 9

 real    0m37.426s
 user    1m49.708s
 sys     1m7.805s
 RUN jobs: 10

 real    0m40.149s
 user    2m0.625s
 sys     1m13.054s
 RUN jobs: 11

 real    0m44.515s
 user    2m18.503s
 sys     1m21.783s
 RUN jobs: 12

 real    0m44.393s
 user    2m25.161s
 sys     1m26.875s
 RUN jobs: 13

 real    0m47.298s
 user    2m44.370s
 sys     1m29.611s
 RUN jobs: 14

 real    0m52.647s
 user    3m16.386s
 sys     1m37.780s
 RUN jobs: 15

 real    0m54.757s
 user    3m18.954s
 sys     1m45.547s
 RUN jobs: 16

 real    0m58.655s
 user    3m49.732s
 sys     1m49.191s
 }}}

 Notice how sys time creeps up taking over performance gain on N>4.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9221#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler