[Haskell-cafe] Google Summer of Code - Lock-free data

John Lato jwlato at gmail.com
Fri Mar 30 00:03:51 CEST 2012

Slightly related: I think it would be interesting to compare a
Disruptor-based concurrency communications mechanism and compare it to
e.g. TChans

1.  Disruptor - http://code.google.com/p/disruptor/

> From: Ryan Newton <rrnewton at gmail.com>
>> I think so. Atomically reading and writing a single memory location
>>> (which CAS does) is just a very simple transaction. But using a CAS
>>> instruction should be more efficient, since STM has to maintain a
>>> transaction log and commit transactions, which creates some overhead.
>> Ah, I see. In that case, it may be worthwhile to implement the CAS
>> instruction in terms of STM as well and measure the performance difference
>> this makes for the final data structure. After all, STM is a lot more
>> compositional than CAS, so I'd like to know whether the loss of
>> expressiveness is worth the gain in performance.
> There's one annoying hitch with doing apples-to-apples comparisons here.
> The problem is that CAS falls short of being a hardware-accelerated version
> of a simple transaction (read TVar, (==) against expected value,
> conditionally update TVar), because CAS is based on pointer equality rather
> than true equality.  (eq? rather than equal? for any Schemers out there.)
> For this reason our "Fake" version of CAS -- for older GHCs and for
> performance comparison -- has to use reallyUnsafePointerEquality#:
> http://hackage.haskell.org/packages/archive/IORefCAS/0.2/doc/html/Data-CAS-Internal-Fake.html
> But we do provide a "CASable" type class in that package which is precisely
> for comparing the performance of:
>   - Hardware CAS
>   - Fake CAS -- atomicModifyIORef + ptrEquality
>   - Foreign CAS -- gcc intrinsic + function call

More information about the Haskell-Cafe mailing list