[GHC] #12437: 20% regression in max_bytes_used for T1969

[GHC]

#12437: 20% regression in max_bytes_used for T1969
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        Reporter:  simonmar          |                Owner:  osa1
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by osa1):

 I build 714bebf ("Implement unboxed sums") and it's parent commit. Here's
 the
 difference:

 {{{
 - Parser allocates the same amount
 - Renamer/typechecker allocates    +0.281 mb
 - Desugar allocates                -0.006 mb
 - Simplifier allocates             +0.093 mb
 - CoreTidy allocates               +0.010 mb
 - CorePrep allocates               +0.001 mb
 - CodeGen allocates                -0.647 mb
 }}}

 overall it should allocate less, and we can see it in the RTS output. This
 is the output from the parent:

 ("bytes allocated", "786881504")

 This is from "implement unboxed sums":

 ("bytes allocated", "786256984")

 This is 624,520 less.

 But max_bytes_used is increased:

 ("max_bytes_used", "16536648")
 to
 ("max_bytes_used", "17697744")

 Difference: +1,161,096

 So it seems like allocation-wise this is better but maybe GC is running at
 different times, and causing residence to increase? I'm not sure how to
 interpret this. Allocation is reduced so maybe this is a good thing?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12437#comment:3>
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