Folks,
first of all, I remember someone already mentioned issue with decreased
parallelism of the GHC build recently somewhere but I cann't find it
now. Sorry, for that since otherwise I would use this thread if it was
on this mailing list.
Anyway, while working on SPARC NCG I'm using T2000 which provides 32
threads/8 core UltraSPARC T1 CPU. The property of this machine is that
it's really slow on single-threaded work. To squeeze some perf from it
man really needs to push 32 threads of work on it. Now, it really hurts
my nerves to see it's lazy building/running just one or two ghc
processes. To verify the fact I've created simple script to collect
number of ghc processes over time and putting this to graph. The result
is in the attached picture. The graph is result of running:
gmake -j64
anyway, the average number of running ghc processes is 4.4 and the
median value is 2. IMHO such low number not only hurts build times on
something like CMT SPARC machine, but also on let say a cluster of ARM
machines using NFS and also on common engineering workstations which
provide these days (IMHO!) around 8-16 cores (and double the threads
number).
My naive idea(s) for fixing this issue is (I'm assuming no Haskell file
imports unused imports here, but perhaps this may be also investigated):
1) provide explicit dependencies which guides make to build in more
optimal way
2) hack GHC's make depend to kind of compute explicit dependencies from
(1) in an optimal way automatically
3) someone already mentioned using shake for building ghc. I don't know
shake but perhaps this is the right direction?
4) hack GHC to compile needed hi file directly in its memory if hi file
is not (yet!) available (issue how to get compiling options right here).
Also I don't know hi file semantics yet so bear with me on this.
Is there anything else which may be done to fix that issue? Is someone
already working on some of those? (I mean those reasonable from the list)?
Thanks!
Karel
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