[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 request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Runtime System    |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Description changed by rrnewton:

Old description:

> 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.

New description:

 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.  No 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.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10728#comment:1>
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