Author: Robert Hyatt
Date: 19:24:29 05/22/03
Go up one level in this thread
On May 22, 2003 at 13:43:55, Tom Kerrigan wrote: >On May 21, 2003 at 22:20:57, Robert Hyatt wrote: > >>On May 21, 2003 at 15:48:46, Tom Kerrigan wrote: >> >>>On May 21, 2003 at 13:46:26, Robert Hyatt wrote: >>> >>>>On May 20, 2003 at 13:52:01, Tom Kerrigan wrote: >>>> >>>>>On May 20, 2003 at 00:26:49, Robert Hyatt wrote: >>>>> >>>>>>Actually it _does_ surprise me. The basic idea is that HT provides improved >>>>>>resource utilization within the CPU. IE would you prefer to have a dual 600mhz >>>>>>or a single 1000mhz machine? I'd generally prefer the dual 600, although for >>>>> >>>>>You're oversimplifying HT. When HT is running two threads, each thread only gets >>>>>half of the core's resources. So instead of your 1GHz vs. dual 600MHz situation, >>>>>what you have is more like a 1GHz Pentium 4 vs. a dual 1GHz Pentium. The dual >>>>>will usually be faster, but in many cases it will be slower, sometimes by a wide >>>>>margin. >>>> >>>>Not quite. Otherwise how do you explain my NPS _increase_ when using a second >>>>thread on a single physical cpu? >>>> >>>>The issue is that now things can be overlapped and more of the CPU core >>>>gets utilized for a greater percent of the total run-time... >>>> >>>>If it were just 50-50 then there would be _zero_ improvement for perfect >>>>algorithms, and a negative improvement for any algorithm with any overhead >>>>whatsoever... >>>> >>>>And the 50-50 doesn't even hold true for all cases, as my test results have >>>>shown, even though I have yet to find any reason for what is going on... >>> >>>Think a little bit before posting, Bob. I said that the chip's execution >>>resources were evenly split, I didn't say that the chip's performance is evently >>>split. That's just stupid. You have to figure in how those execution resources >>>are utilized and understand that adding more of these resources gives you >>>diminishing returns. >>> >>>-Tom >> >> >>You shold follow your own advice. If resources are split "50-50" then how >>can _my_ program produce a 70-30 split on occasion? >> >>It simply is _not_ possible. >> >>There is more to this than a simple explanation offers... > >Now you're getting off onto another topic here. > Read backward. _I_ did not "change the topic". I said that I don't see how it is possible for HT to slow a program down. You said "50-50" resource allocation might be an explanation. I said "that doesn't seem plausible because I have at least one example of two compute-bound threads that don't show a 50-50 balance on SMT." If Eugene is right, and I don't know as he was not sure and I haven't read anything similar to what he mentioned, that _could_ explain it (ie if some resources are split 50-50 between the two logical processors even if one could use more than the other due to the particular application being run. However that seems like a _bad_ design decision if it is true...) However there are probably other plausible explanations as well. What is the _real_ explanation? That will likely take some time to figure out. >Originally you were saying that it's impossible for HT to slow a program down >unless there was something wrong with the algorithm. And based on testing here, I pretty well stick with that. I won't say there is _no_ program that will run slower, but I haven't found one myself. And again, to be clear, we are talking about one program, one thread. Run on a machine with SMT on and SMT off. I've run that test repeatedly and can't find any penalty for one thread when turning SMT on. ANd I do mean _no penalty_ on anything I have tried. Kernel builds. Compiles. Running Crafty. Running various compute-bound applications like NAMD, a big monte-carlo simulation, etc... The idea really doesn't make sense, IMHO. > >Now you're back to complaining about your 70-30 split, which is only related to >the original topic because they both involve ratios like "50-50" and "70-30." That 70-30 was used simply to suggest that 50-50 is _not_ a "golden rule" in SMT resource allocation, apparently. Nothing more. > >-Tom
This page took 0.02 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.