Author: Tom Kerrigan
Date: 13:04:27 05/24/03
Go up one level in this thread
On May 24, 2003 at 01:13:39, Robert Hyatt wrote: >On May 23, 2003 at 23:51:54, Tom Kerrigan wrote: > >>On May 23, 2003 at 22:58:51, Robert Hyatt wrote: >> >>>On May 22, 2003 at 23:29:25, Aaron Gordon wrote: >>> >>>>On May 22, 2003 at 22:24:29, Robert Hyatt wrote: >>>> >>>>>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 >>>> >>>> >>>>Hyatt, grab distributed.net's RC5-72 client, it supports multiple cpus and with >>>>every dual system I've seen it run it on gets an exact 100% increase in >>>>nodes/second. Now, it only spawns 1 thread per processor & isn't memory >>>>intensive what so ever (that i've seen, only CPU clock speed affects results). A >>>>P4 with HT gets HALF the speed of a P4 w/o HT in some of the results I've seen, >>>>if you get the time try to verify that for me. I would have figured this would >>>>have been one of the programs HT would shine at. Complete surprise to me... If >>>>you could, grab the linux RC5-72 client at: >>> >>>What are they measuring? >>> >>>IE running two copies _should_ see each copy run about 1/2 as fast with SMT >>>on, since each copy is getting roughly 50% of available cpu core resources >>>when running the same instruction streams. >> >>Huh, looks like Hyatt _can_ learn something... Still wrong, but closer. >> >>-Tom > > >Now if _you_ would only do the same... Heh. What am I going to learn from you? All you do is write posts about how you don't know what's going on. -Tom
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