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

[GHC]

#9221: (super!) linear slowdown of parallel builds on 40 core machine
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        Reporter:  carter            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.8.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #910, #8224       |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Sergei Trofimovich <siarheit@…>):

 In [changeset:"f93c363fab1ac8ce6f0b474f5967b0b097995827/ghc"
 f93c363f/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f93c363fab1ac8ce6f0b474f5967b0b097995827"
 extend '-fmax-worker-args' limit to specialiser (Trac #11565)

 It's a complementary change to
     a48de37dcca98e7d477040b0ed298bcd1b3ab303
     restore -fmax-worker-args handling (Trac #11565)

 I don't have a small example but I've noticed another
 discrepancy when was profiling GHC for performance

     cmmExprNative :: ReferenceKind -> CmmExpr -> CmmOptM CmmExpr

 was specialised by 'spec_one' down to a function with arity 159.
 As a result 'perf record' pointed at it as at slowest
 function in whole ghc library.

 I've extended -fmax-worker-args effect to 'spec_one'
 as it does the same worker/wrapper split to push
 arguments to the heap.

 The change decreases heap usage on a synth.bash benchmark
 (Trac #9221) from 67G down to 64G (-4%). Benchmark runtime
 decreased from 14.5 s down to 14.s (-7%).

 Signed-off-by: Sergei Trofimovich <siarheit at google.com>

 Reviewers: ezyang, simonpj, austin, goldfire, bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2507

 GHC Trac Issues: #11565
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9221#comment:68>
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