[GHC] #13701: GHCi 2x slower without -keep-tmp-files

[GHC]

#13701: GHCi 2x slower without -keep-tmp-files
-------------------------------------+-------------------------------------
           Reporter:  niteria        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHCi           |           Version:  8.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In D3562, I've observed that -keep-tmp-files makes :load 3x faster on my
 test case.
 I can't share my test case, but I've found a way to approximate it with
 `MultiLayerModules` I just added in D3575.

 Here are the steps:
 {{{
 # in ghc top dir
 $ mkdir tmp
 $ cd tmp
 $ cp ../testsuite/tests/perf/compiler/genMultiLayerModules .
 # edit genMultiLayerModules to say DEPTH=0, WIDTH=5000
 $ ./genMultiLayerModules
 $ echo ':load MultiLayerModules' | ../inplace/bin/ghc-stage2 --interactive
 +RTS -s
   11,132,224,952 bytes allocated in the heap
    1,004,238,408 bytes copied during GC
      185,091,216 bytes maximum residency (14 sample(s))
        2,813,504 bytes maximum slop
              365 MB total memory in use (0 MB lost due to fragmentation)

                                      Tot time (elapsed)  Avg pause  Max
 pause
   Gen  0       706 colls,     0 par    0.907s   0.906s     0.0013s
 0.0125s
   Gen  1        14 colls,     0 par    0.607s   0.606s     0.0433s
 0.2244s

   TASKS: 4 (1 bound, 3 peak workers (3 total), using -N1)

   SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

   INIT    time    0.001s  (  0.000s elapsed)
   MUT     time   20.219s  ( 20.493s elapsed)
   GC      time    1.514s  (  1.513s elapsed)
   EXIT    time    0.000s  (  0.005s elapsed)
   Total   time   21.733s  ( 22.010s elapsed)

   Alloc rate    550,585,275 bytes per MUT second

   Productivity  93.0% of total user, 93.1% of total elapsed
 $ echo ':load MultiLayerModules' | ../inplace/bin/ghc-stage2 --interactive
 -keep-tmp-files +RTS -s
    4,603,831,672 bytes allocated in the heap
      971,623,904 bytes copied during GC
      184,019,808 bytes maximum residency (14 sample(s))
        2,262,680 bytes maximum slop
              365 MB total memory in use (0 MB lost due to fragmentation)

                                      Tot time (elapsed)  Avg pause  Max
 pause
   Gen  0       448 colls,     0 par    0.724s   0.723s     0.0016s
 0.0321s
   Gen  1        14 colls,     0 par    0.621s   0.620s     0.0443s
 0.2242s

   TASKS: 4 (1 bound, 3 peak workers (3 total), using -N1)

   SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

   INIT    time    0.001s  (  0.000s elapsed)
   MUT     time    7.966s  (  8.202s elapsed)
   GC      time    1.345s  (  1.344s elapsed)
   EXIT    time    0.000s  (  0.004s elapsed)
   Total   time    9.312s  (  9.550s elapsed)

   Alloc rate    577,938,762 bytes per MUT second

   Productivity  85.5% of total user, 85.9% of total elapsed
 }}}

 So it's 2x slower and allocates 2.5x more.

 Profiling pointed to
 https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/main/SysTools.hs;8bf50d5026f92eb5a6768eb2ac38479802da1411$1074
 We're creating `dont_delete_set` a lot.

 Looks like this was improved in D3111 recently.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13701>
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