Author: Ernst A. Heinz
Date: 11:15:35 11/13/97
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On November 13, 1997 at 13:49:48, Robert Hyatt wrote: > >B.T. Fraise had a 533mhz alpha running on ICC. He was getting around >150K >nodes per second with Crafty, which he runs there as "Data". Jason used >a >plain vanilla DEC alpha/500mhz in Paris, and we were getting around 250K >nodes per second. He compared that machine to a P6/200 using the same >OS (NT) and was getting around 80K. So the /500 gave us 250/80 speedup >which is a factor of 3.1 > >Data went from 80K to 150K, which was less than 2.0 speedup. Even >worse, >on his alpha, we saw *dramatic* speed variations, from 40K to 150K, >running >the *same* test position multiple times. It seems that there is some >cache >condition that can be badly broken, or *something*. Because we did not >see >this wild fluctuation on the Digital machines. > >main point of all this is that a cheap alpha might be a "cheap alpha". >We >need some data to compare all of these machines, and find out why some >are >dogs and others are vipers. If Fraise uses Linux and "gcc" --> mystery solved! DEC's compilers and optimizers that power DEC Unix and also Visual C++ for Windows/NT on Alpha produce *much* better code than "gcc". Especially the new ones that already allow for dedicated EV56 tuning. Furthermore, "Crafty" seems to be quite memory-bound which makes it much more sensitive to inferior/superior L3 cache and main memory design as for latency and throughput. The AlphaStation 500 that Crafty ran on in Paris featured 2MB L3 cache and 256MB of fast SDRAM. What does Fraise have? As for the speed differences between multiple program invocations, I stand by my earlier conclusion that it is related to "bad" mapping of VM pages to physical memory pages resulting in subsequent L1 I-cache thrashing. We see this phenomenon for "DarkThought" on *all* Alpha-based systems up to now -- including the cooled 767MHz speed demon we ran on in Paris. =Ernst=
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