On 8 October 2016 at 17:55, Ben Gamari
lonetiger@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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