[GHC] #8732: Global big object heap allocator lock causes contention

[GHC]

#8732: Global big object heap allocator lock causes contention
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        Reporter:  tibbe                    |            Owner:  simonmar
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:  7.10.1
       Component:  Runtime System           |          Version:  7.6.3
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  Runtime performance bug  |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by simonmar):

 I don't really understand why in mighty he couldn't just re-use the same
 block.

 I'm kind of surprised that this is a bottleneck, and I think it needs more
 investigation.  We only take the lock for large objects, so typically
 there's going to be a lot of computation going on per allocation.

 I suppose if it really is a problem then we could just have a per-thread
 block pool at the granularity of a megablock to avoid fragmentation
 issues.  We just push the global lock back to the megablock free list.
 This has the danger that we might have a lot of free blocks owned by one
 thread that don't get used, though, so we might want to redistribute the
 free blocks at GC.  Things start to get annoyingly complicated.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8732#comment:9>
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