Author: Tom Kerrigan
Date: 14:24:24 05/19/03
Go up one level in this thread
On May 18, 2003 at 20:17:12, Robert Hyatt wrote: >On May 18, 2003 at 12:50:32, Aaron Gordon wrote: > >>On May 18, 2003 at 11:53:57, Robert Hyatt wrote: >> >>>On May 17, 2003 at 20:50:52, Aaron Gordon wrote: >>> >>>>On May 17, 2003 at 15:21:53, Jeremiah Penery wrote: >>>> >>>>>On May 16, 2003 at 07:04:02, Aaron Gordon wrote: >>>>> >>>>>>On May 15, 2003 at 21:35:35, Nolan Denson wrote: >>>>>> >>>>>>>http://www6.tomshardware.com/cpu/20030513/index.html >>>>>> >>>>>>Look at the seti@home benchmarks, tell me nothing looks screwy there... >>>>>>P4-3GHz w/ the same L2 & bus/memory speed as the P4-2.8 yet it 'magically' does >>>>>>it in half the time? Riiiggghhhtt.. :) >>>>> >>>>>That would probably be because of Hyperthreading. >>>> >>>>Seti doesn't support multiple threads, and hyperthreading won't increase >>>>performance 100%, especially for Seti@home. >>> >>> >>>For the right instruction mix it will double cpu speed. Check out Eugene's >>>comments about running the tablebase compression utility (two copies) on a >>>single SMT-enabled CPU. This application apparently has just the right >>>memory reference pattern and two copies will run in the same time that one >>>will run... >>> >>>That's the only example I have heard of, but I haven't done a lot of checking >>>to see if there are others... >> >>What I was saying was that I've done seti for years now, and have done the >>testing and know the results tom posted are impossible. Also, other than what >>you & Eugene have stated I haven't seen HT help out more than 20% and in most >>cases it hurts performance, which is also what AMD came up with. >> >>http://www.theinquirer.net/?article=9475 > >I don't buy the "hurts performance" at all. Any more than I would buy the >statement that two processors are slower than one. In _rare_ cases it is >true, but in the general or average case it is not true. > >I've not found _any_ code here that does worse with SMT on than with it off. > >Of course if you run one app with SMT off, then run the same app with SMP >on and run something _else_ with it, it will run slower, but that's a stupid >test paradigm... Of course, if you run a single-threaded app, it will run exactly as fast with HT on or off. (The processor only gets divided when there are 2 threads to run.) If you're testing an app like a web server or a database which will detect how many processors you have and start threads accordingly, then it shouldn't surprise you if HT runs slower. Algorithmic inefficiencies or resource contention can both cause stuff to run slower. I remember all the initial HT benchmarks from 3rd parties showed that turning HT on slowed down enterprise stuff (servers and whatnot) by a couple %. (Enterprise stuff was tested initially because HT was turned on for Xeons first.) I don't buy for a second AMD's claim that turning HT on slows your machine down on average, though. -Tom
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