Default options for -threaded

Matthew Pickering matthewtpickering at gmail.com
Fri Jan 4 15:21:40 UTC 2019


Two years seems a good amount of time for any objectors.

https://ghc.haskell.org/trac/ghc/ticket/16126#ticket

On Fri, Oct 21, 2016 at 5:35 PM Simon Marlow <marlowsd at gmail.com> wrote:
>
> On 8 October 2016 at 17:55, Ben Gamari <ben at well-typed.com> wrote:
>>
>> lonetiger at gmail.com writes:
>>
>> > Hi All,
>> >
>> > A user on https://ghc.haskell.org/trac/ghc/ticket/11054 has asked why
>> > -N -qa isn’t the default for -threaded.
>> >
>> I'm not sure that scheduling on all of the cores on the user's machine by
>> default is a good idea, especially given that our users have
>> learned to expect the existing default. Enabling affinity by default
>> seems reasonable if we have evidence that it helps the majority of
>> applications, but we would first need to introduce an additional
>> flag to disable it.
>
>
> Affinity is almost always a bad idea in my experience.
>
>>
>> In general I think -N1 is a reasonable default as it acknowledges the
>> fact that deploying parallelism is not something that can be done
>> blindly in many (most?) applications. To make effective use of
>> parallelism the user needs to understand their hardware, their
>> application, and its interaction with the runtime system and configure
>> the RTS appropriately.
>>
>
> Agree on keeping -N1.
>
> Related to this, I think it's about time we made -threaded the default.  We could add a -single-threaded option to get back the old behaviour.
>
> There is a small overhead to using -threaded, but -threaded is also required to make a lot of things work (e.g. waitForProcess in a multithreaded program, not to mention parallelism).
>
> Anyone interested in doing this?
>
> Cheers
> Simon
>
>
>>
>> Of course, this is just my two-cents.
>>
>> Cheers,
>>
>> - Ben
>>
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