Author: Vincent Diepeveen
Date: 12:59:42 07/10/03
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On July 09, 2003 at 19:21:52, Jeremiah Penery wrote: >On July 09, 2003 at 08:25:39, Vincent Diepeveen wrote: > >>Nevertheless this machine is record breaking and always will be remembered for >>that. Assuming it is designed for big vectors it's quite a bit slower in latency >>then because if you optimize for huge transfers at once then a single transfer >>is probably very pricey. >> >>So let's ignore the latency question, it wasn't designed for it simply. > >Instead of just guessing, why don't you go look it up. Information is widely >available. > >Here - http://www.sc.doe.gov/ascr/dongarra.pdf - the MPI_PUT latency is listed >as 6.63us. Everywhere else I've seen lists under 10us, with most being much >closer to 5us. When it goes to some central bottleneck then you never can avoid such huge latencies. For a supercomputer that can do OpenMP till 2048 processors like the Earth machine (if i interpret data in that pdf well) then at 500Mhz with 16 instructions a clock (therefore called vector processor) which also can be achieved actually at 8 gflops it is really a great machine for the matrix guys. Most likely that 6.63 latency us is for huge lines of data as they achieve 12.xxGB a second with it. Note that MPI_PUT is a one way function. It isn't *waiting* for data to get back. However if we consider circumstances and the design of the stuff out there that really isn't interesting. Interesting is that they can get 12.xx GB bandwidth with MPI_PUT. This stuff is not designed for chessprograms. So if we are busy with just getting random cache lines of say 128 bytes at most, then the latency will be more around 20 us at this machine. That's not nice to say however as it is not designed for this. It is designed to put 12.8GB through the central router with a MPI_PUT a second and that is an incredible achievement for node to node. Best regards, Vincent
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