[darcs-users] darcs patch: Add --debug-http flag to enable curl and... (and 3 more)

[Simon Marlow]

Dmitry Kurochkin wrote:
> On Thu, Aug 14, 2008 at 4:43 AM, Gwern Branwen <gwern0 at gmail.com> wrote:
>> Actually, while we're on the topic: why does URL.hs now use forkOS? Darcs uses forkIO, usually, which is the standard Haskell practice. It can't be for performance: I replaced forkOS with forkIO and tested it with 4 darcs gets of the Tahoe repo, and they all clocked in at 24-27 minutes (the same as with forkOS).
> 
> That is because we do foreign calls that block. AFAIK if we use
> unbound threads (forkIO) all haskell threads are halt during blocking
> syscalls from foreign functions. And that is not what we want.

This is a common misconception: forkIO vs. forkOS makes no difference with 
respect to blocking.  forkOS is only necessary if you need to call a C 
library that makes use of thread-local state, otherwise it just uses extra 
resources and will slow things down: context-switching between a forkOS 
thread and another thread constitutes a full OS-thread context switch, 
which is much much slower than switching between forkIO threads.

People don't seem to believe me when I say this.  In retrospect we 
shoulnd't have called it forkOS, we should have called it 
forkReallyExpensiveOnlyNecessaryForCallingOpenGL_IO.

Cheers,
	Simon