Re: If you had a month to improve gcc build parallelization, where would you begin?

[Jeff Law <law at redhat dot com>]

On 04/03/2013 10:00 PM, Geert Bosch wrote:
> This will be true regardless of communication method. There is so little
> opportunity for parallelism that anything more than 4-8 local cores is
> pretty much wasted. On a 4-core machine, more than 50% of the wall time
> is spent on things that will not use more than those 4 cores regardless.
> If the other 40-50% or so can be cut by a factor 4 compared to 4-core
> execution, we still are talking about at most a 30% improvement on the
> total wall time. Even a small serial overhead for communicating sources
> and binaries will still reduce this 30%.
For stage2 & stage3 optimized gcc & libstdc++, I'd tend to disagree (oh 
yea, PCH generation is another annoying serialization point). 
Communication kills on things like libgcc, libgfortran, etc where the 
bits we're compiling at any given time are trivial.

I haven't tested things in the last couple years, but I certainly had to 
keep the machines roughly in the same class performance-wise.  It's a 
real killer if insn-attrtab.o gets sent to the slowest machine in the 
cluster.

And having an over-sized cluster allows the wife & kids to take over 
boxes without me ever really noticing (unless I've got multiple builds 
flying).


> We need to improve the Makefiles before it makes sense to use more
> parallelism.  Otherwise we'll just keep running into Amdahl's law.
Can't argue with that.  We're still leaving significant stuff on the 
floor due to lameness in our makefiles.

jeff