On 21 May 2015, at 15:54, Bardur Arantsson wrote:
> fork/exec is almost certainly going to be negligable compared to the
> overall compile time anyway. It's not like GHC is fast enough for it to
> matter.
Don't count on it. On our Windows desktop machines, fork/exec costs
approximately one third of a second, instead of the expected small number of
milliseconds or less. The reasons are unknown, but we suspect a misconfigured
anti-virus scanner (and for various company policy reasons we are prohibited
from doing the investigation that could confirm or deny this hypothesis).
This means that when ghc --make does lots of external things requiring a fork,
such as preprocessing, a medium sized project (using many library packages) can
take a surprisingly large amount of time (minutes instead of seconds), even for
an incremental build where very little code has changed. We think an
in-process cpphs could make some of our compilations literally hundreds of
times faster.
Regards,
Malcolm
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