On 21/06/2011 10:02, Herbert Valerio Riedel wrote:
Hello Simon,
On Fri, 2011-06-17 at 09:05 +0100, Simon Marlow wrote:
What's happening there? The actual processing work seems to be done in a single HEC... but what are the remaining 11 HECs doing exactly? Am I doing something wrong?
The answer is, they're all doing GC. When you say -N, the parallel GC is turned on, so every GC requires the cooperation of all cores. When you're running parallel code this is a big win, but for sequential code it could well be a slowdown.
Speaking about cooperation of all cores... how much is the parallel GC affected by "multitasking-noise" (is there a better name for it?) in the system?
There are two cases I'm thinking about:
a) Say, I have a 8-core desktop workstation and run my GC-intensive (or massively parallel processing) Haskell program with "+RTS -N8", but I have a few desktop apps running, and using up a bit of CPU time (but on average just a few %)
Does this already cause significant (i.e. measurable) synchronization delays due to 'non fully dedicated cores' in my Haskell program?
Yes it can do, although since we started using 'yield' in the spinlock code the problem is less noticeable. I've been trying to address this problem with a new GC, for details see this paper: http://community.haskell.org/~simonmar/papers/local-gc.pdf There are a couple of workarounds if you're badly affected: - use a larger -A setting. This might also degrade performance due to more cache misses (try it and see). If your processor has lots of cache you might be able to go to -A1m or -A2m which reduces the GC frequency without impacting cache behaviour. - Don't use all the cores - e.g. use -N7 on an 8-core.
b) What about virtualized guests (e.g. with VMware, KVM, etc)? Let's assume the Host system has 16 cores, and I partition those into 2 Guest VMs with 8 cores each assigned; Will there be a measurable/significant slow-downs due to synchronization delays in my "+RTS -N8" Haskell program?
I haven't tried with a VM, it would be an interesting experiment! Cheers, Simon