[GHC] #10587: Suspending and unsuspending ghci kills and spawns threads

[GHC]

#10587: Suspending and unsuspending ghci kills and spawns threads
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        Reporter:  niteria       |                   Owner:
            Type:  bug           |                  Status:  new
        Priority:  low           |               Milestone:
       Component:  GHCi          |                 Version:  7.10.1
      Resolution:                |                Keywords:
Operating System:  Linux         |            Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |               Test Case:
      Blocked By:                |                Blocking:
 Related Tickets:                |  Differential Revisions:
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Changes (by niteria):

 * owner:  niteria =>


Comment:

 I've tried to change it, so that the number of IO manager threads gets
 decreased when number of capabilities decreases, but I couldn't come up
 with a solution that satisfied me. The main problem is that the thread is
 not the only resource associated with the IO manager, there are also
 callback that should be executed in the future. If we stopped the thread
 before its loop is empty, we would break the promise of executing that
 callback.

 This problem is also described here:
 https://phabricator.haskell.org/rGHC12f3fef5ec52c1ec0958b674adcd981f48048428


 I've considered:

 * Checking if the loop is empty - I couldn't tell if all the
 implementations support that, so I abandoned that idea
 * Migrating the file descriptors and callbacks to another instance of IO
 manager - the problem is that it isn't obvious which one to migrate to and
 if we'd want to spread them across available ones.

 @simonmar suggested just being less aggressive about killing spare
 workers. I'm starting to like that idea.

 I'm out of ideas and it's not really that urgent for me, so I'll suspend
 working on this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10587#comment:3>
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