Re: Threading the compiler

[Mike Stump <mrs at apple dot com>]

On Nov 10, 2006, at 5:43 PM, Paul Brook wrote:
> Can you make it run on my graphics card too?

:-)  You know all the power on a bleeding edge system is in the GPU  
now.  People are already starting to migrate data processing for  
their applications to it.  Don't bet against it.  In fact, we hide  
such migration behind apis that people already know and love, and you  
might be doing it in your applications already, if you're not  
careful.  And before you start laughing too hard, they are doubling  
every 12 months, we've only managed to double every 18 months.  Let's  
just say, the CPU is doomed.

> Seriously thought I don't really understand what sort of response  
> you're expecting.

Just consensus building.

> Do you have any justification for aiming for 8x parallelism in this  
> release and 2x increase in parallelism in the next release?

Our standard box we ship today that people do compiles on tends to be  
a 4 way box.  If a released compiler made use of the hardware we ship  
today, it would need to be 4 way.  For us to have had the feature in  
the compiler we ship with those systems, the feature would have had  
to be in gcc-4.0.  Intel has already announced 4 core chips that are  
pin compatible with the 2 core chips.  Their ship date is in 3 days.   
People have already dropped them in our boxes and they have 8 way  
machines, today.  For them to make use of those cores, today, gcc-4.0  
would had to have been 8 way capable.  The rate of increase in cores  
is 2x every 18 months.  gcc releases are about one every 12-18  
months.  By the time I deploy gcc-4.2, I could use 8 way, by the time  
I stop using gcc-4.2, I could make use of 16-32 cores I suspect.  :-(

> Why not just aim for 16x in the first instance?

If 16x is more work than 8x, then I can't yet pony up the work  
required for 16x myself.  If cheap enough, I'll design a system where  
it is just N-way.  Won't know til I start doing code.

> You mention that "competition is already starting to make  
> progress". Have they found it to be as easy as you imply?

I didn't ask if they found it easy or not.

> whole-program optimisation and SMP machines have been around for a  
> fair while now, so I'm guessing not.

I don't know of anything that is particularly hard about it, but, if  
you know of bits that are hard, or have pointer to such, I'd be  
interested in it.