Author: Robert Hyatt
Date: 07:31:02 12/17/02
Go up one level in this thread
On December 17, 2002 at 10:10:46, Vincent Diepeveen wrote: >Hello, > >Some tests were performed in the USA, where some P4 Xeon dual 2.8Ghz >systems get delivered now. In Europe we can't get them yet and >most likely we don't want them either: > >Here are the results of DIEP at the Xeon 2.8Ghz dual ECC registered DDR ram. > >test 1: diep 4 processes. Of course HT enabled. > 181538 nps > >test 2: diep 2 processes. HT enabled. > 135924 nps > >test 3: diep 2 processes K7 1.6ghz (registered DDR ram all other settings > identical to xeon dual setup): > 146555 > >THE 2 TESTS NALIMOV DIDN'T OR COULDN'T WANT TO DO WITH CRAFTY >SOME WEEKS AGO REVEAL A BIG WEAKNESS OF HT/SMT: > >test 4: diep 2 processes. HT disabled. 171288 nps > >test 5 and 6: diep single cpu HT disabled and enabled were same speed > 92090 nps versus 92019 nps. > >First conclusion is that the system is profitting only from HT when you >use 4 processes at the same time, OTHERWISE IT IS A DISADVANTAGE IF >YOU MULTITHREAD, because see the big difference between 2 processes >running with HT turned on and off. So? _some_ of us are smart enough to understand this even before trying to run it. It has _already_ been discussed here. > >In itself when you have a program with just 2 threads which you >run on a dual it gets slower. My assumption is that the hardware reports >4 cpu's and that the software doesn't care at what cpu to schedule >the processes/threads. the result of that is that there is a 33% chance >that things get scheduled at a cpu which is already running a thread/process. It doesn't necessarily get slower. And once you move to windows .new, or the new linux kernel gets fixed, this won't happen at all as both of these systems will understand that two threads need to run on two physical processors rather than on two logical processors on the same physical processor... what is your point? This is a sudden revellation to you? :) > >Resulting in a system where 1 cpu idles kind of shortly and 1 cpu is running >2 threads/processes. > >Actually the actual chance that the 2 processes are scheduled at >2 different processors (there is 4 processors for the OS >times 3 processors left for the second process is 12 different >schedulings) is: 8/12 = 2/3 = 66%. In short there is a disaster possibility >of 33%. Until you move to a newer operating system that understands hyper-threading. This is a known problem. Windows .net supposedly has this fix already. Linux is "in progress". _not_ a big deal. A _known_ issue for months now... > >Now the absolute speed from performance viewpoint. If the system idles >completely and then starts to run *exclusively* diep at 4 processors, then >the measured speedup as you can calculate is in the order of 11.4% for >SMT/HT. > >That's not so much actually. The loss by searching parallel is at most >parallel applications bigger than the win of 11.4%. In case of DIEP >i am on the lucky side and go for that 11.4% faster speed. Can't be. If you get 11.4% speed increase, and you lose more than that in parallel search overhead, you have a problem on _normal_ SMP machines as well. > >Yet the sad confirmation is that the pessimistic expectation about the >absolute speed is completely confirmed. This system performs (assuming >lineair scaling) like a 1.98 Ghz dual K7. > >there are motherboards now which do not require registered memory and >the K7 runs already quite a while at 2.0Ghz in fact. Now i don't care >for XP at all here nor do i care for the P4 at all. I just care for >parallel search here. > >If we know that a 2.0Ghz dual K7 is identical to a dual 2.8Ghz Xeon >and that in the majority of cases the K7 is going to win, then considering >the huge price difference, the choice would be trivial for most who >are looking for a lot of computing power for little money. > >Doesn't take away the fact that the P4 is winning ground. I remember >the first dual AMD 1.2ghz test versus P4 dual 1.7Ghz and the AMD dual >being 20% faster. Meaning in short that the speed of a P4 was performing >about 1 : 1.7 > >Now if i compare a dual Xeon 2.8Ghz with a 2Ghz K7 then it's equal >meaning the P4 is performing 1 : 1.4 > >So that's a big step forward! This from someone that said these machines "do not exist" a week ago, too. So this is a _big_ step forward alright. :) > >Whether the step is because of DDR ram versus the very bad performing >RDRAM (nearly 2 times slower latency) is a matter of open discussion. > >HT/SMT in itself is not so impressing now. > >It's trivial to say that it will get impressive when the P4 can split itself >into 2 real processors having little dependencies on each other. > >Right now the single cpu win on a P4 3.06Ghz HT (18%) is >clearly more than the older generation 2.8 Ghz HT/SMT. so it seems >also this technique is slowly winning in realism. > >Right now i can't take what's getting on the market now very serious. > >Best regards, >Vincent
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