On Tue 2008-09-09 12:30, Bruce Eckel wrote: > So this is the kind of problem I keep running into. There will seem to be > consensus that you can do everything with isolated processes message passing > (and note here that I include Actors in this scenario even if their mechanism > is more complex). And then someone will pipe up and say "well, of course, you > have to have threads" and the argument is usually "for efficiency."
Some pipe up and say ``you can't do global shared memory because it's inefficient''. Ensuring cache coherency with many processors operating on shared memory is a nightmare and inevitably leads to poor performance. Perhaps some optimizations could be done if the programs were guaranteed to have no mutable state, but that's not realistic. Almost all high performance machines (think top500) are distributed memory with very few cores per node. Parallel programs are normally written using MPI for communication and they can achieve nearly linear scaling to 10^5 processors BlueGene/L for scientific problems with strong global coupling. I encourage you to browse these slides for some perspective on very large scale coupled computation. Most problems commercial/industrial tasks are much easier since the global coupling is much looser. http://www.xergi.no/upload/IKT/9011/SimOslo/eVITA/2008/PetaflopsGeilo.pdf Jed
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