[GHC] #13288: Resident set size exceeds +RTS -M limit with large nurseries

[GHC]

#13288: Resident set size exceeds +RTS -M limit with large nurseries
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           Reporter:  j6carey        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Runtime        |           Version:  8.0.2
  System                             |
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Runtime
  (amd64)                            |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 We observed high resident set size well in excess of the `+RTS -M` limit
 in a long-running, high data-volume Haskell application that Awake
 Networks is deploying on a network appliance.

 We think that the current `GC.c` code has two bugs that, at least in
 combination with each other, become significant when high `+RTS -N` and
 very high `+RTS -A` values are used.

 1. As we approach the `-M` limit, the computation of the new size for
 generation 1 appears to be based on an incorrect figure for the total size
 of the nursery.  The `-A` value is used instead of the product of that
 value with `-N`.  This problem could lead to the total heap size exceeding
 the `-M` limit.

 2. Memory allocated from the operating system is freed only if the RTS
 thinks that it would not be reallocated soon.  The estimate for what will
 be needed soon is based on fewer inputs than the actual resizing logic,
 and in particular it is not affected by `-M`.  Thus it might keep free
 mblocks in excess of the `-M` limit, based on an expected heap growth that
 would be forbidden by `-M`.

 We are preparing a patch to address these issues; it will point out the
 particular lines of code.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13288>
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