On 17/08/2010 06:09, Gregory Collins wrote:
Does GHC expose any primitives for things like atomic compare-and-swap?
I can't seem to find anything in the docs. I'm wondering if it's
possible, for example, to implement things like the wait-free concurrent
queue from [1] or a lock-free wait-free hash table like the Azul people
are doing [2].
We could provide a compare-and-swap on IORefs, indeed I've been planning
to try that so we could move the implementation of atomicModifyIORef
into user-space, so to speak.
Anecdotally it's hard to beat the performance of a good pure data
structure in a mutable container. For example, the mutable list
experiments we did a while back [1] were all at least a factor of 10
slower than IORef [a].
[1] http://www.haskell.org/~simonmar/bib/concurrent-data-08_abstract.html
In network servers, we often have a large number of clients fighting
over shared resources -- even simple stuff like "which threads are
scheduled to timeout?" and "which threads are currently active, so I can
kill them if I need to?". Shared mutable collections seem to be a fact
of life here.
Under load, pure data structures behind MVars don't perform well because
of lock contention, and in my experience atomicModifyIORef is marginally
better but still suffers from contention issues (probably related to
thunk blackholing).
This is the specific case that we addressed in the GHC runtime recently,
and you should find that 6.14 will perform a lot better than 6.12 with
pure data structures in mutable containers, especially with
atomicModifyIORef.
In the meantime you'll probably get better performance using STM.
Whether you should perform the update strictly inside the transaction or
not is a matter for experimentation, it probably depends on the data
structure.
Recently I got a ~35% speedup on a benchmark by switching one of these
tables from a Map behind an IORef to a hashmap using a striped-locking
scheme (see [3] if you're interested.) I think there's a need for
high-concurrency data structures like this, and I don't see much on
Hackage that addresses it. As much as we like to say that we have a
handle on concurrency issues, from what I can see java.util.concurrent.*
has us soundly thrashed in this department, especially as it relates to
exposing atomic wait-free CPU primitives like the ones in
java.util.concurrent.atomic.* [4]. Speaking as a partisan, this cannot
be allowed to stand, can it?
Absolutely. Although the Java crowd have to grapple with a complicated
memory model which makes building these data structures virtually
impossible. At least in Haskell you can whip up a concurrent version of
any pure data structure trivially, and have a lot of confidence that you
got it right. Of course you could do that in Java, but they don't tend
to build pure data structures by default, whereas we have them by the
bucketload.
Cheers,
Simon
Cheers,
G.
------------------------------------------------------------------------------
[1] Maged M. Michael and Michael L. Scott, "Simple, Fast, and Practical
Non-Blocking and Blocking Concurrent Queue Algorithms":
http://www.cs.rochester.edu/u/michael/PODC96.html
[2] http://www.stanford.edu/class/ee380/Abstracts/070221_LockFreeHash.pdf
[3]
http://github.com/snapframework/snap-server/blob/master/src/Data/HashMap/Concurrent.hs#L1
[4]
http://download.oracle.com/javase/6/docs/api/java/util/concurrent/atomic/AtomicInteger.html
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
