Author: Matt Taylor
Date: 10:22:47 12/17/02
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On December 17, 2002 at 12:09:22, Gian-Carlo Pascutto wrote: >On December 17, 2002 at 12:03:38, Matt Taylor wrote: > >>Actually I based it on data that Dr. Hyatt posted previously. The data Vincent >>has for his program doesn't show such wonderful gains. > >Ok - first see my above post; I looked at the wrong log. I don't >have data for this exact comparison yet, so you may be right - or not. > >I don't trust any data that is produced by either of the two >so I prefer to run my own tests. > >>>>but it's been optimized for HT. >>> >>>It's not - even Robert will tell you this. >> >>Ok, it's been optimized for Pentium 4, which is -almost- the same thing. If it >>runs well on P4, it runs well with HT because it will fall into an I/O burst >>cycle. > >Can you explain this last sentence? > >-- >GCP I/O burst cycle is a concept from operating systems. Programs do a bit of work and then they do some form of I/O. It makes sense; you click your mouse, some code determines what you clicked and what to do in response, and then it spits it back out at you in the form of output. You can think of memory accesses also occuring according to an I/O burst cycle, though it is less pronounced. Generally, the CPU needs data to be in its registers to manipulate it. Code will load its data, say a matrix, do the manipulations, and store the results. It's like a miniature I/O burst cycle. The point I am making is that one thread will be busy doing I/O (which is slow) while another thread gets to do real work. They'll alternate like that. Ideally you get a 100% speed-up from HT because their cycles "hug" each other -- one comes out of I/O just as the other goes into I/O. You don't get 100% speed-up in most HT cases because that doesn't happen very often. I was wrong, however, as I was under the impression that Eugene had put some hand-tweaked code into the version that ran those benchmarks. -Matt
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