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

[Segher Boessenkool]

>> How does that work?
>> The binaries have to get the all the machines of the clusters  
>> somewhere.
>> Does this assume you are using NFS or similar for your build  
>> directory?
>> Won't the overhead of using that instead of local disk kill most  
>> of the
>> parallelization benefit of a cluster over a single SMP machine?
>
> 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%.
>
> We need to improve the Makefiles before it makes sense to use more
> parallelism.  Otherwise we'll just keep running into Amdahl's law.

Some numbers, 16-core 64-thread POWER7, c,c++,fortran bootstrap:

-j6:
real    57m32.245s
user    205m51.480s
sys     6m24.043s

-j10:
real    45m55.034s
user    211m37.833s
sys     6m33.305s

-j15:
real    41m51.061s
user    237m26.174s
sys     7m2.341s

-j60:
real    38m18.583s
user    336m12.393s
sys     11m26.717s


Segher