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On 08/04/2011 11:51 PM, Dan Zuras Intervals wrote:
From: James Demmel<demmel@xxxxxxxxxxxxxxxxx> Just to supply a little more background on the need for being able to get the same answer when you run your program more than once: Many of you may recall a post to NA-Digest a couple of years ago, in which a commercial FEM software developer was asking whether anyone knew of a "repeatable" parallel sparse linear system solver; here "repeatable" means getting the same answer when you type a.out twice on the same machine, not the harder problem of the same answer on different machines.
It makes sense to require repeatability on the same machine. I expect thst, too, for debugging purposes.
But the prior discussion was about repeatability across platforms, which can hardly be guaranteed without specifying exactly the requires result of each operation (including rounding errors), and hence is an unreasonable requirement.
The point of all this is to say that repeatability on the same machine (let alone on different machines) is both widely expected and desired, hard to attain, and likely to be an unpleasant surprise to many users if and when they realize this. This is true with or without intervals. Of course interval bounds that are reliably narrow, if not repeatable, will mitigate the problem.