#10728: Add e.g. "-N<=4" in addition to the fixed "-N4" and variable "-N" RTS options -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- 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. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10728#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler