On 2008-09-10, David Roundy
On Wed, Sep 10, 2008 at 03:30:50PM +0200, Jed Brown wrote:
On Wed 2008-09-10 09:05, David Roundy wrote:
I should point out, however, that in my experience MPI programming involves deadlocks and synchronization handling that are at least as nasty as any I've run into doing shared-memory threading.
Absolutely, avoiding deadlock is the first priority (before error handling). If you use the non-blocking interface, you have to be very conscious of whether a buffer is being used or the call has completed. Regardless, the API requires the programmer to maintain a very clear distinction between locally owned and remote memory.
Even with the blocking interface, you had subtle bugs that I found pretty tricky to deal with. e.g. the reduce functions in lam3 (or was it lam4) at one point didn't actually manage to result in the same values on all nodes (with differences caused by roundoff error), which led to rare deadlocks, when it so happened that two nodes disagreed as to when a loop was completed. Perhaps someone made the mistake of assuming that addition was associative, or maybe it was something triggered by the non-IEEE floating point we were using. But in any case, it was pretty nasty. And it was precisely the kind of bug that won't show up except when you're doing something like MPI where you are pretty much forced to assume that the same (pure!) computation has the same effect on each node.
Ah, okay. I think that's a real edge case, and probably not how most use MPI. I've used both threads and MPI; MPI, while cumbersome, never gave me any hard-to-debug deadlock problems. -- Aaron Denney -><-