Duncan Coutts wrote:
On Wed, 2008-04-30 at 21:01 -0400, Norman Ramsey wrote:
Dear Fellow GHC Hackers,
I'll be buying new computers soon, and right now the only thing I do
for which computer performance is a problem is GHC hacking---especially
running 'validate'. I've got a good performance boost on my current
machine by using two SATA drives striped in a RAID-0 configuration.
I'm wondering what other tricks may be warranted. Specifically
I'm interested in answers to the following questions:
1. How much benefit does one get from four cores versus two?
I know that the build process runs in parallel thanks to
make -j, but I don't know how much parallelism is available
in the testing part. Maybe somebody who already has
quad core can compare and let us know the results?
I recently got a quad core amd64 and I run linux. I noticed from the
system monitor graphs that not that much of the build process can use
many cores. I was using make -j5.
The one bit that really does benefit is building the ghc compiler
itself. That can make full use of all 4 cores for an extended period.
The ./configure step is painfully serial and building the early tools
and the libraries is rather serial too. There are lots of little changes
here that we could do to make it more parallel. One example is that we
link 20 copies of Setup.hs rather than reusing one copy. Linking is very
much serial.
In principle the test suite should be possible to parallelise but I
don't know much about python and parallelism.
We're pretty close to being able to run the testsuite in parallel, but
we ran up against a Python bug:
http://hackage.haskell.org/trac/ghc/ticket/1558
perhaps we could work around that bug with a big lock or something.
Unfortunately it only happens a handful of times over a testsuite run,
so it's hard to work on.
This is the low-hanging fruit for speeding up validate right now.
Anyone feel like having a crack at it?
Cheers,
Simon
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