[GHC] #9370: large blowup in memory usage and time when doing parallel build of xmlhtml package

[GHC]

#9370: large blowup in memory usage and time when doing parallel build of xmlhtml
package
-------------------------------------+-------------------------------------
       Reporter:  carter             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 I've observed a blowup in ghc memory usage when invoked with parallel
 build flags.

 to reproduce

 {{{
 cabal get xmlhtml-0.2.3.2
 cabal install xmlhtml-0.2.3.2 --only-dependencies
 cd xmlhtml-0.2.3.2
 }}}

 then
 {{{
  cabal clean ; cabal configure --ghc-options="-j4" ; time cabal build -j4
 -v3
 }}}
 will take quite a while and use > 1gb of ram

 whereas

 {{{
  cabal clean ; cabal configure --ghc-options="-j1 ; time cabal build -j1
 -v3

 }}}

 will use  < 150mb of ram.

 Based upon the output  of cabal build -j4 -v3, it looks like the parallel
 build is spending a lottt more time in the simplifier  passes.  I'm not
 sure what changes  between the parallel and sequential builds to change
 this. I'll try to dig into this more in a few days, but recording this
 problem now before i forget. (though of course any insights would be
 appreciated)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9370>
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