Default options for -threaded

[Ben Gamari]

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.

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.

Of course, this is just my two-cents.

Cheers,

- Ben
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