[GHC] #10728: Add e.g. "-N<=4" in addition to the fixed "-N4" and variable "-N" RTS options

[GHC]

#10728: Add e.g. "-N<=4" in addition to the fixed "-N4" and variable "-N" RTS
options
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              Reporter:  rrnewton    |             Owner:  simonmar
                  Type:  feature     |            Status:  new
  request                            |
              Priority:  normal      |         Milestone:
             Component:  Runtime     |           Version:  7.10.2
  System                             |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 As discussed in this issue:

    https://github.com/commercialhaskell/stack/issues/680

 stack is one example of a program that optimistically turned on "-N".
 Indeed, it feels like a reasonable and even safe option for a
 multithreaded program.  Unfortunately, -N currently guarantees bad
 performance on large machines and especially on large machines with
 hyperthreading.  Pretty much one should ship an executable with -N by
 default as of GHC 7.8 and 7.10 IMHO.

 Unfortunately, even if stack did get a speedup at, say, 4 or 8 cores, it
 would not be good to ship it with "-N8" either.  This would be an
 unreasonable choice on small, one or two core machines.

 What we need is a way to say that the program can productively use
 parallelism up to a certain upper bound, but that fewer threads should be
 used if there are not enough cores available.  I propose "-N<=8" as a
 potential syntax.

 Currently, this behavior can be achieved with
 `setNumCapabilities`/`getNumProcessors`, but I think it's worth a command
 line RTS option.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10728>
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