[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:"a5d26f26d33bc04f31eaff50b7d633444192b4cb/ghc" a5d26f2/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="a5d26f26d33bc04f31eaff50b7d633444192b4cb"
 rts: enable parallel GC scan of large (32M+) allocation area

 Parallel GC does not scan large allocation area (-A)
 effectively as it does not do work stealing from nursery
 by default.

 That leads to large imbalance when only one of threads
 overflows allocation area: most of GC threads finish
 quickly (as there is not much to collect) and sit idle
 waiting while single GC thread finishes scan of single
 allocation area for that thread.

 The patch enables work stealing for (equivalent of -qb0)
 allocation area of -A32M or higher.

 Tested on a highlighting-kate package from Trac #9221

 On 8-core machine the difference is around 5% faster
 of wall-clock time. On 24-core VM the speedup is 20%.

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

 Test Plan: measured wall time and GC parallelism on highlighting-kate
 build

 Reviewers: austin, bgamari, erikd, simonmar

 Reviewed By: bgamari, simonmar

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2483

 GHC Trac Issues: #9221
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9221#comment:59>
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