[Haskell-cafe] Re: Support for lock-free/wait-free programming?
marlowsd at gmail.com
Tue Aug 17 06:07:00 EDT 2010
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  or a lock-free wait-free hash table like the Azul people
> are doing .
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  were all at least a factor of 10
slower than IORef [a].
> 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
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
> 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  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.* . 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
>  Maged M. Michael and Michael L. Scott, "Simple, Fast, and Practical
> Non-Blocking and Blocking Concurrent Queue Algorithms":
>  http://www.stanford.edu/class/ee380/Abstracts/070221_LockFreeHash.pdf
>  http://github.com/snapframework/snap-server/blob/master/src/Data/HashMap/Concurrent.hs#L1
>  http://download.oracle.com/javase/6/docs/api/java/util/concurrent/atomic/AtomicInteger.html
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