[GHC] #15938: Hadrian's recompilation check is extremely slow

[GHC]

#15938: Hadrian's recompilation check is extremely slow
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.8.1
       Component:  Build System      |              Version:  8.6.2
  (Hadrian)                          |
      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 tdammers):

 `+RTS -s` output when starting on a half-built tree; kept hadrian running
 until the first proper compilations happen ("Run Ghc CompileHs ..."):

 {{{
   16,130,616,376 bytes allocated in the heap
    1,080,660,392 bytes copied during GC
      149,548,072 bytes maximum residency (12 sample(s))
        1,189,912 bytes maximum slop
              339 MB total memory in use (0 MB lost due to fragmentation)

                                      Tot time (elapsed)  Avg pause  Max
 pause
   Gen  0     14677 colls,     0 par    0.952s   1.016s     0.0001s
 0.0022s
   Gen  1        12 colls,     0 par    0.532s   0.640s     0.0533s
 0.1704s

   TASKS: 5 (1 bound, 4 peak workers (4 total), using -N1)

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

   INIT    time    0.000s  (  0.000s elapsed)
   MUT     time    9.544s  ( 13.735s elapsed)
   GC      time    1.484s  (  1.656s elapsed)
   EXIT    time    0.000s  (  0.009s elapsed)
   Total   time   11.028s  ( 15.400s elapsed)

   Alloc rate    1,690,131,640 bytes per MUT second

   Productivity  86.5% of total user, 89.2% of total elapsed
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15938#comment:25>
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