On 02/11/2009 21:49, Henry DeYoung wrote:
On Mon, November 2, 2009 9:09 am, Simon Marlow wrote:
On 31/10/2009 18:27, Henry DeYoung wrote:
We've decided to use oprofile to sample cache misses recorded by the
performance counters. To get reasonable information out of oprofile,
I'd
like to compile the nofib benchmarks via C with export of debugging
symbols.
As a first step toward this, I've changed the NoFibHcOpts variable in
nofib/mk/boilerplate.mk to be:
NoFibHcOpts = -O -fvia-C -pgmc gcc -optc-g
That won't work, I'm afraid. The Evil Mangler script that
post-processes the .s file output by gcc does not understand the debug
annotations added by gcc's -g flag.
The best you can do with oprofile is get assembly-level profiling. It's
not too bad actually; I've used it in the past myself, although oprofile
itself I found to be a bit flaky - for some reason it seems to stop
counting randomly on my laptop.
You might have better luck using the new performance counter support
("performance events") in Linux 2.6.31. I've not been able to test it
yet myself because apparently it doesn't support the CPU in my laptop, but
I plan to find another machine to try it on soon.
So now I'm using
NoFibHcOpts = -O -keep-s-files -opta-g
which seems to work fine. You mentioned that assembly-level profiling is
the best you can do with oprofile (in particular). Is there another
profiler that would allow source-level (in Haskell or the generated C
code) profiling, or is this as good as it gets for any profiler since GHC
doesn't seem to export debugging symbols at the source level?
This is as good as it gets, yes.
Also, based on what you've heard, would you recommend the Linux 2.6.31
performance counter tools over the PAPI library? (I see that there is
PAPI support in the GHC runtime:
http://hackage.haskell.org/trac/ghc/wiki/PAPI)
The main difficulty with PAPI is that it is so hard to get it working.
You need a patched kernel, and then building and installing PAPI itself
is not at all easy. If I was going to install it on a new machine, I'd
set aside a day to do it.
The new Linux performance events infrastructure will be in the default
kernel that comes with most distributions - Ubuntu Karmic includes it
for example. That makes it a lot more attractive for me. Also it has
support for annotating source (or assembly) with profiling results, like
oprofile. I don't think PAPI has that, but I could be wrong. On the
other hand, PAPI has an API for saving/restoring sets of counters, which
is what we use in GHC's RTS for separating the GC counters from the
mutator. I hope I'll be able to do that with perf events, but I don't
know yet.
Thanks so much for all your help!
Cheers,
Simon
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