Re: gcc 3.5 integration branch proposal

[Mike Stump <mrs at apple dot com>]

On Tuesday, January 20, 2004, at 11:03 AM, Alexandre Oliva wrote:
> Is the compiler server multi-threaded?

Nope.

> Even if it were, Mosix wouldn't get anything from it since threads 
> that run on a shared memory space don't migrate to different hosts.  
> At least last I
> looked, they didn't.

Right.  And since we don't (currently) share memory spaces, there isn't 
a problem.

> Sure, if you start multiple compile servers, they can migrate to
> different boxes.  Ditto for distcc builds.  But then each one
> duplicates some of the compilation that the compiler server is
> supposed to save, so by distributing the load you diminish the
> benefits of the sharing.

Yes, this is true.  The problem is, if the CPUs are idle, then in the 
amount they wait, waiting for one CPU to compute an answer, they could 
have computed it themselves and started compilation.  In addition to 
computing the answer, it would have to be shipped over the network as 
well, which just requires more resources.  But, yes, long term, it does 
make for an interesting research project.  What level of granularity is 
appropriate...  and how to manage it.

> > We do distcc and PCH today, perfectly compatible as well.
>
> How can you do PCH with distcc, if distcc just ships the preprocessed
> sources to the remote box?

:-)  You'd be interested in our magic -fpch-preprocess option.  It lets 
you ship a file that is 100x smaller, and requires 6x less processing.  
Check out http://gcc.gnu.org/ml/gcc/2003-03/msg00369.html and 
http://gcc.gnu.org/ml/gcc-patches/2003-03/msg00499.html  Anyway, 
eventually I'll submit it again...  Maybe in the coming year it might 
get in, here's to hoping.  We've been shipping it for a while now.

> Usage of distcc simply disables PCH for
> me.  Are you hiding any patches from the rest of us, that embed PCH
> data in the preprocessor output?

Yes.

> This might be nice, but it would probably be yet another incompatible 
> change in the preprocessor output format that might cause problems to 
> other tools.

No, it is fairly safe and not the default.  It does require that distcc 
know to put on one extra option, but I thought that was best, too many 
users of -E otherwise.