Author: Matt Taylor
Date: 08:27:18 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. Crafty gets better results with HT, but it's been optimized for HT. It just means you need a personal Intel engineer to make it blazing fast for people who plopped down $600 USD for a top-of-the-line Intel chip. Before long they'll start selling Intel engineers in local computer shops. Collect all 18... HT is a good idea, and it works in practice rather than just on paper. It just doesn't work for -everything-. >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. > >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. > >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%. Yes, when one thread is scheduled on one processor, there are 3 choices for the other thread, and one is disaster. 1/3 = 33%. >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. > >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. If memory is a big issue for Diep, it probably won't scale linearly as memory never does. >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. AMD has always been better price/performance. Before the huge price differences in AMD and Intel chips, the AMD chips meant your old Socket 7 board could be used through ~500 MHz. >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! Well just about every application saw a similar gain from the 512 KB cache Northwood from the 256 KB cache Williamette. The new Xeons, as I understand, have 1 MB L3 cache in -addition- to the other caches. Don't quote me there. All I know is that things changed. The extra cache makes the P4 competitive whereas before P4 performance was something of an oxymoron, a joke among the people who'd seen its scores, and a disappointment for former Intel fans. You'll probably observe the trend shift (not -completely-) toward the former when AMD releases Barton, likewise equipped with 512 KB of L2 cache. >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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