Author: Robert Hyatt
Date: 10:05:48 12/17/02
Go up one level in this thread
On December 17, 2002 at 11:27:18, Matt Taylor wrote: >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... Not sure where that comes from. First, the first SMT/HT report came from Eugene. With a _stock_ crafty. Without the pause. I have been playing with the pause and have discovered that it doesn't make a lot of difference, on the 1-2% range at best... > >HT is a good idea, and it works in practice rather than just on paper. It just >doesn't work for -everything-. In Vincent's world, if it doesn't work for _him_ it doesn't work, period... > >>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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