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

Sebastian Sylvan sebastian.sylvan at gmail.com
Thu Dec 15 08:17:18 EST 2005

On 12/14/05, Simon Marlow <simonmar at microsoft.com> wrote:
> 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.

Wouldn't there be a speedup to do both writes and waiting at the array
level, BUT annotated with an index? So a write to an array would only
wake threads with the appropriate index.
I'm not up to snuff on the implementation details here, so maybe this
is what the Array-of-Tvars would reduce to when compiled...

Anyway, the main gist of my original post was that TArrays should be
in the libraries, so that I can safely use it without having to send
along my own implementation each time (and potentially colliding with
someone else's implementation down the line). When/if a primitve
TArray is implemented, the Array-of-Tvars-approach could just be
replaced, and all programs which use the TArray would get an automatic


Sebastian Sylvan
UIN: 44640862

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