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Subject: SURPRISING RESULTS P4 Xeon dual 2.8Ghz

Author: Vincent Diepeveen

Date: 07:10:46 12/17/02


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.

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%.

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.

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!

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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