Dual CPU perfomance hit

bayanne · 2002-08-16, 07:29

I know that it has been discussed at length in the past, but when a dual cpu has both processors running an LL test each exactly what perfomance hit is seen?

I was just thinking that if you have one cpu on an LL test and another factoring, then as the factoring credit is so much less (10%?), it would make more sense to have both cpu doing LL tests even if the perfomance dipped by 20% for each cpu.

Anyone have any thoughts?

Digital Concepts · 2002-08-16, 22:07

Does facotring take 10% of the time of a LL?

If that is the case, then you wouldn't want a 20% decrease by running two LLs.

Stormblade · 2002-08-16, 23:40

well, it depends.. the bottleneck on multiprocessor systems is memory bandwith. Each processor is attempting to constantly access memory. On a multipipeline rambus system, I found I could run 2 instances LL testing with very little performance hit. any more than that and all instances dropped drastically (took twice as long for each processor to finish)

When factoring, the code will fit in the cache of most modern processors, so it doesnt need to access memory very much. Therefore you are in fact getting twice as much work done.. (man, if we could just set it up so that one processor would factor an exponent, and then pass it to the second processor for LL test would be keen.. :) )

bayanne · 2002-08-17, 08:24

Sorry, I can't have explained myself very well.

More than once in the past I have read that factoring gets about 1/10th the p90 cpu years credit that perfoming an LL/DC would get.

My question was that if you only get 10% of the cpu yrs credit for running a second processor on factoring whilst the other is running on an LL test, then it must make better sense to take the slight performance hit you get from both cpu on LL tests.

Or can someone substantiate this better with some benchmarks and cpu yrs credited on a multi cpu setup.

I do think that is an important statistic to be known as the number of multi cpu boards is increasing, and we want to be able to use them to crunch at their maximum cpu yr throughput.

Xyzzy · 2002-08-17, 13:06

I think what bayanne means is this...

Pretend you have two computers... Each runs for one month... One is running a LL test... The other is running a factoring test... After a month, the LL computer gets 5 years of credit and the factoring one gets ½ year of credit... This is because the credit is weighted to give the LL test more credit...

Anyways, everyone suggests that you run one LL test and one factoring test on a dual CPU system, because running a factoring test on the second CPU causes no performance drop. If you do this, after a month you would have 5.5 years of credit...

But, if the second CPU runs a LL test at ½ speed, due to contention for memory bandwidth, you would get 7.5 years... That is 5 years for the first CPU and 2½ years for the second... Or maybe 3¾ years for each of them...

So it would make more sense to run 2 LL tests even though there is a bottleneck...

IMO, it would be cheaper and more sensible to run two single processor boxes... Then you would get 10 years...

Or maybe I am looking at it wrong?

bayanne · 2002-08-17, 19:11

:D :D

Digital Concepts · 2002-08-18, 04:45

We have never confirmed that time spent factoring is weighted less - hence my question if it gets 10% credit is it because it takes 10% of the time.

Confusion came from one of George's pages that I think we probably misinterpreted. If you ask me now, I'd say that the 10 to 1 ratio mentioned, was probably more about how to climb up the ranks early in the game than about credit weighting.

bayanne · 2002-08-18, 07:38

From the FAQ held at heretics site
http://ws9.jobnegotiator.com/index.html
in the section entitled Assignments, Testing and Test Results one of the questions is:
"I have been told that you get 1/10th the credit for factoring work. Is this true?" http://ws9.jobnegotiator.com/html/faqs/faq21.html#036
to which the answer is: "You are correct on the credit for factoring work. "

Could George maybe clarify what the position is?

binarydigits · 2002-08-18, 15:07

I have been tracking the performance of my machines, and I have determined that the ratio of TF to LL credit is about 70% on most of them, although IIRC it is slightly worse for the athlons than for P2s or P3s. (That is, a machine that will do 3 P90 years per month LL testing will only do about 2.1 yrs/mo trial factoring.) On a P4 it is much worse, at least before v22. But on a K6 it is reversed; the ratio is about 160%, due to the lack of a decent FPU for LL testing. My 475MHz-K7 was running LL tests slower than my 233Mhz-PPro; now I have them both trial factoring and the K7 is almost twice as fast as the PPro. I wish I had known that all along!

xtreme2k · 2002-08-18, 15:19

Quote:

Originally Posted by Digital Concepts

...hence my question if it gets 10% credit is it because it takes 10% of the time.

From my understanding of whats on the mersenne site, this is how it is. I could be wrong but thats how I take it from reading the site.

Say if Factoring took your Pentium 90 30days of CPU time to complete. Only 3 days of that 30 will be credited. It will be as if it took your CPU 3 days to complete the factoring. Therefore, 3 days of CPU time added instead of 30 in the database.

Mivacca2 · 2002-09-06, 20:34

I have a quick question about dual CPU's. You can smack me around and call me whatever, but I was interested in something that intel is going to do in the near*(being relative) future with their desktop processors. They are going to put hyperthreading into them, which i am giong to imagine someone is going to build a motherboard that uses dual P4's on it. I know this would seriously put a dent in the Xenon/Itaniuam area seeing as how the only advantage is that it will be able tto address an obscene amount of memory. So enough of that and to my quesion is hyperthreading between multiple CPU's going to affect speeds any? I would imagine so since the processors are working on the same processes and instructions, but then again i don't have much experience with dual CPU's so any insight would be greatly appreciated.
Thanks,
Mivacca2
(I thought about starting a new thread, but there was already one here about dual CPU's so...)

2002-08-16, 07:29	#1
bayanne "Tony Gott" Aug 2002 Yell, Shetland, UK 373 Posts	Dual CPU perfomance hit I know that it has been discussed at length in the past, but when a dual cpu has both processors running an LL test each exactly what perfomance hit is seen? I was just thinking that if you have one cpu on an LL test and another factoring, then as the factoring credit is so much less (10%?), it would make more sense to have both cpu doing LL tests even if the perfomance dipped by 20% for each cpu. Anyone have any thoughts?

2002-09-06, 20:34	#11
Mivacca2 Aug 2002 12₈ Posts	Hyper Threading I have a quick question about dual CPU's. You can smack me around and call me whatever, but I was interested in something that intel is going to do in the near*(being relative) future with their desktop processors. They are going to put hyperthreading into them, which i am giong to imagine someone is going to build a motherboard that uses dual P4's on it. I know this would seriously put a dent in the Xenon/Itaniuam area seeing as how the only advantage is that it will be able tto address an obscene amount of memory. So enough of that and to my quesion is hyperthreading between multiple CPU's going to affect speeds any? I would imagine so since the processors are working on the same processes and instructions, but then again i don't have much experience with dual CPU's so any insight would be greatly appreciated. Thanks, Mivacca2 (I thought about starting a new thread, but there was already one here about dual CPU's so...)

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2002-08-16, 22:07	#2
Digital Concepts Aug 2002 66₈ Posts	Does facotring take 10% of the time of a LL? If that is the case, then you wouldn't want a 20% decrease by running two LLs.

2002-08-16, 23:40	#3
Stormblade Aug 2002 2·5 Posts	well, it depends.. the bottleneck on multiprocessor systems is memory bandwith. Each processor is attempting to constantly access memory. On a multipipeline rambus system, I found I could run 2 instances LL testing with very little performance hit. any more than that and all instances dropped drastically (took twice as long for each processor to finish) When factoring, the code will fit in the cache of most modern processors, so it doesnt need to access memory very much. Therefore you are in fact getting twice as much work done.. (man, if we could just set it up so that one processor would factor an exponent, and then pass it to the second processor for LL test would be keen.. :) )

2002-08-17, 08:24	#4
bayanne "Tony Gott" Aug 2002 Yell, Shetland, UK 373 Posts	Sorry, I can't have explained myself very well. More than once in the past I have read that factoring gets about 1/10th the p90 cpu years credit that perfoming an LL/DC would get. My question was that if you only get 10% of the cpu yrs credit for running a second processor on factoring whilst the other is running on an LL test, then it must make better sense to take the slight performance hit you get from both cpu on LL tests. Or can someone substantiate this better with some benchmarks and cpu yrs credited on a multi cpu setup. I do think that is an important statistic to be known as the number of multi cpu boards is increasing, and we want to be able to use them to crunch at their maximum cpu yr throughput.

2002-08-17, 13:06	#5
Xyzzy Aug 2002 2·3²·13·37 Posts	I think what bayanne means is this... Pretend you have two computers... Each runs for one month... One is running a LL test... The other is running a factoring test... After a month, the LL computer gets 5 years of credit and the factoring one gets ½ year of credit... This is because the credit is weighted to give the LL test more credit... Anyways, everyone suggests that you run one LL test and one factoring test on a dual CPU system, because running a factoring test on the second CPU causes no performance drop. If you do this, after a month you would have 5.5 years of credit... But, if the second CPU runs a LL test at ½ speed, due to contention for memory bandwidth, you would get 7.5 years... That is 5 years for the first CPU and 2½ years for the second... Or maybe 3¾ years for each of them... So it would make more sense to run 2 LL tests even though there is a bottleneck... IMO, it would be cheaper and more sensible to run two single processor boxes... Then you would get 10 years... Or maybe I am looking at it wrong?

2002-08-17, 19:11	#6
bayanne "Tony Gott" Aug 2002 Yell, Shetland, UK 565₈ Posts	:D :D

2002-08-18, 04:45	#7
Digital Concepts Aug 2002 2·3³ Posts	We have never confirmed that time spent factoring is weighted less - hence my question if it gets 10% credit is it because it takes 10% of the time. Confusion came from one of George's pages that I think we probably misinterpreted. If you ask me now, I'd say that the 10 to 1 ratio mentioned, was probably more about how to climb up the ranks early in the game than about credit weighting.

2002-08-18, 07:38	#8
bayanne "Tony Gott" Aug 2002 Yell, Shetland, UK 373₁₀ Posts	From the FAQ held at heretics site http://ws9.jobnegotiator.com/index.html in the section entitled Assignments, Testing and Test Results one of the questions is: "I have been told that you get 1/10th the credit for factoring work. Is this true?" http://ws9.jobnegotiator.com/html/faqs/faq21.html#036 to which the answer is: "You are correct on the credit for factoring work. " Could George maybe clarify what the position is?

2002-08-18, 15:07	#9
binarydigits Aug 2002 64₈ Posts	I have been tracking the performance of my machines, and I have determined that the ratio of TF to LL credit is about 70% on most of them, although IIRC it is slightly worse for the athlons than for P2s or P3s. (That is, a machine that will do 3 P90 years per month LL testing will only do about 2.1 yrs/mo trial factoring.) On a P4 it is much worse, at least before v22. But on a K6 it is reversed; the ratio is about 160%, due to the lack of a decent FPU for LL testing. My 475MHz-K7 was running LL tests slower than my 233Mhz-PPro; now I have them both trial factoring and the K7 is almost twice as fast as the PPro. I wish I had known that all along!