[Haskell-cafe] Support for lock-free/wait-free programming?

Gregory Collins greg at gregorycollins.net
Tue Aug 17 01:09:41 EDT 2010

Hello all,

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].

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).

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?




[1] Maged M. Michael and Michael L. Scott, "Simple, Fast, and Practical
Non-Blocking and Blocking Concurrent Queue Algorithms":

[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

Gregory Collins <greg at gregorycollins.net>

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