[Haskell-cafe] Re: Real-time garbage collection for Haskell
cjs at starling-software.com
Tue Mar 9 00:04:05 EST 2010
On 2010-03-06 12:42 +0000 (Sat), Simon Marlow wrote:
> Usually I find keeping the nursery size (-A) close to the L2 cache size
> works best, although sometimes making it really big can be even better.
Interesting to know. I got the impression that I was being encouraged to
keep -A closer to the L1 cache size, myself.
> -qg disables parallel GC completely. This is usually terrible for
> locality, because every GC will move all the recently allocated data
> from each CPU's L2 cache into the cache of the CPU doing GC, where it
> will have to be fetched out again after GC.
I've since explained to Cranshaw (we are getting to have *way* too many
'Simon's around here) about the issues with our different machines; some
of this depends on the host on which we're doing the testing.
* Our Core i7 hosts share 8 MB of L3 cache amongst four cores with
two threads each. Thus, no locality penalties here.
* Our Xeon E5420 host has two 4-core CPUs, and each pair of cores
shares a 6 MB L2 cache. Thus there's a pretty good chance that
something you need is in someone else's cache. I don't know if
there's any difference between moving stuff between two caches on
the same CPU and two caches on different CPUs.
* Our Xeon E5520 host has two 4-core CPUs, each core of which has
two threads. Each CPU (4 cores) shares an 8 MB L3 cache. Thus,
presumably, less locality penalty than the E5420 but more than an
i7. As a side note, I also see slightly less memory bandwidth on
this system (for both caches and main memory) than I do on an i7.
This gets complex pretty fast. And don't ask me about Intel's new style
of having L1 and L3 or L2 and L3 caches rather than L1 and L2 caches.
> -qb disables load-balancing in the parallel GC, which improves locality
> at the expense of parallelism, usually I find it is an improvement in
> parallel programs.
I'd think so too. Figuring out what went on here is going to have to
wait until I get more detailed GC information in the eventlog.
Followups to glasgow-haskell-users at haskell.org.
Curt Sampson <cjs at cynic.net> +81 90 7737 2974
The power of accurate observation is commonly called cynicism
by those who have not got it. --George Bernard Shaw
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