[Haskell-cafe] RE: [Haskell] TArray?

Simon Marlow simonmar at microsoft.com
Wed Dec 14 04:36:05 EST 2005

On 13 December 2005 14:52, Jan-Willem Maessen wrote:

> On Dec 13, 2005, at 8:46 AM, Simon Marlow wrote:
>> [In response to another plea for TArrays]
>> In the past I have used arrays of TVars, as Thomasz suggested.  It
>> would indeed be better to have a primitive STM array, the only
>> problem with this is the extra complexity.  One simplifying
>> assumption is that it should consider changes at the level of the
>> whole array, rather than per-element (otherwise you'd use an array
>> of TVars). 
> Actually, in that case it might be more useful to have a TMVar
> containing an array.  But I suspect the need for this use case is
> small.  I know a ton of uses for transactionally-updated arrays for
> which the goal is to permit concurrent access to independent array
> elements (concurrent hash tables come to mind as an obvious use case
> where transactions make life vastly simpler).
> You might ask Tim Harris whether there's a reasonably simple, clever
> way to do this
> using arrays + CAS.  I believe such a trick exists---you might end up
> waking too many threads on a write, but you'd get read/write
> concurrency at least.

One simple plan is to log writes at the element level during a
transaction (i.e. don't copy the whole array), but don't track *waiting*
at the element level, so a thread waiting on a TArray will get woken up
whenever the array is modified.  You get read/write concurrency this
way.  Committing a transaction still has to lock the whole array during
the update.

Tim, do you know of any better schemes?


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