Did I read a post re 4 cores to NOT LL all?

[petrw1]

I seem to recall reading on this forum that on a Quad it is not the best use of the PC to do LL tests on all 4 cores; something about overhead on the CPU? or RAM? I think it recommended doing TF on one core?

If I am not imagining things then can someone tell me what the conditions were? i.e. Does it depend on the CPU or RAM Technology or Speed or the OS?

I have an Intel Q9550 (Quad core 2.83 Ghz)
with 4GB DDR1066 RAM
and Vista 64.

[Uncwilly]

Originally Posted by petrw1

I seem to recall reading on this forum that on a Quad it is not the best use of the PC to do LL tests on all 4 cores; something about overhead on the CPU? or RAM? I think it recommended doing TF on one core?

If I am not imagining things then can someone tell me what the conditions were? i.e. Does it depend on the CPU or RAM Technology or Speed or the OS?

The limitations in accessing RAM (irrespective of tech.) cause the multiple threads to compete and choke down the through put. With a quad, I personally would suggest at least 1, maybe 2 threads doing T-F. T-F puts less demand on the bus. Others may offer their own suggestions based upon actual experience, but that is my dos centavos.

[petrw1]

You may be right....I swapped one core to TF and the other three doing LL (though still in the P-1 phase) are now running at least 10% faster

[Phantomas]

Hi petrw1,

I think you should give the new v25.8 a try with AffinityScrambling set to AffinityScramble=1230 an 2 worker / 2 helper threads for LL-Tests

see http://mersenneforum.org/showpost.ph...7&postcount=21

[Kevin]

With your memory, there's not as much of a performance hit as most people get when you run LL tests on all 4 cores. I think it's along the lines of with DDR2-800 or below, you only get something like 3 cores worth of output, but with DDR2-1066 it's closer to 3.5 cores worth of output. It's not big enough of a hit to discourage me from running LL on all 4 cores of my two quad-cores.

Whether it's "better" to run LL on all 4 cores or LL-TF-LL-TF is somewhat a matter of opinion. If you just care about total production, then with the current crediting system you're best off alternating. Back when TF only got credited 1/10 as much, it was worth the performance hit to run LL on all 4 cores. If you care about advancing the project, then I believe that LL testing is the way to go. I'm pretty sure the crediting disparity was originally created to motivate LL testing over factoring, and I think with that disparity gone we're going to eventually see the TF leading edge pull away from the LL testing leading edge.

[petrw1]

Originally Posted by Phantomas

Hi petrw1,

I think you should give the new v25.8 a try with AffinityScrambling set to AffinityScramble=1230 an 2 worker / 2 helper threads for LL-Tests

see http://mersenneforum.org/showpost.ph...7&postcount=21

Do I understand correctly that this will give me 2 concurrent LL tests with 2 cores jointly working on each?

And if I do this will the Per Iteration time be close to half so that whether I do 4 LL tests on seperate cores OR 2 by 2 cores each the total elapsed time for 4 tests will be about the same?

[Phantomas]

Originally Posted by petrw1

Do I understand correctly that this will give me 2 concurrent LL tests with 2 cores jointly working on each?

Yes, that's right. And when I interpret my results right, than each LL will run on one DualCore, so (my impression) one LL can use the 6MB L2 Cache alone, an it doesn't need to access the ordinary RAM so often.

Originally Posted by petrw1

And if I do this will the per Iteration time be close to half so that whether I do 4 LL tests on seperate cores OR 2 by 2 cores each the total elapsed time for 4 tests will be about the same?

This is the case in my test with my Q9450. With 4 independent LL-Tests (2560K) one itteration is about 54.somewhat ms. With 2 LL-Tests with 2 cores it's about 26.somewhatelse ms. So it is in fact a little, tiny bit faster. And I assume that this is because it's using the L2 Cache better. My RAM runs at 1200MHz 6,6,6,15, and maybe the effect is bigger on 800MHz Ram's (hope so...)

But it seems to be important to run one test on Core [1,2], and the other on core [0,3]. Else my itteration time went up 20%.

[sdbardwick]

For future reference, all of the above applies to Intel quad core processors prior to the i7 series. Most of the adjustments/tweaks listed probably do not need to be done on i7 systems (at least those with triple channel RAM) and won't have a significant effect on Phenom quad cores.

[Phantomas]

For future reference, all of the above applies to Intel quad core processors

Yepp!

[ADBjester]

Originally Posted by Kevin

With your memory, there's not as much of a performance hit as most people get when you run LL tests on all 4 cores. I think it's along the lines of with DDR2-800 or below, you only get something like 3 cores worth of output, but with DDR2-1066 it's closer to 3.5 cores worth of output. It's not big enough of a hit to discourage me from running LL on all 4 cores of my two quad-cores.

The bandwidth of the RAM itself is not the limiting factor. The limiting factor is the memory bus itself, and contention for it by four cores. 800 Mhz QDR RAM is more than enough for what we do. Moving to 1066 Mhz won't add any more than 5% to your performance.

Of far greater concern to him is the chipset... as the nVidia chipsets have far greater problems with all four cores demand high volume access to the memory bus, whereas the Intel chipsets are much, MUCH better.

If he's running nVidia, 2 LL and 2 TF are about optimal. If he's running Intel, then 4 LL are fine -- downshifting to 3 LL and 1 TF doesn't buy you any improvement.

Jester

[S485122]

Originally Posted by ADBjester

The bandwidth of the RAM itself is not the limiting factor. The limiting factor is the memory bus itself, and contention for it by four cores. 800 Mhz QDR RAM is more than enough for what we do. Moving to 1066 Mhz won't add any more than 5% to your performance.

I do not agree, on P4 D and on Core2 Quad the performance of Prime95 was proportional to the memory speed (measured from 533 MHz DDR2 to 1066 MHz DDR2.)

Originally Posted by ADBjester

... the nVidia chipsets have far greater problems with all four cores demand high volume access to the memory bus, whereas the Intel chipsets are much, MUCH better.

Yes

Originally Posted by ADBjester

If he's running nVidia, 2 LL and 2 TF are about optimal. If he's running Intel, then 4 LL are fine -- downshifting to 3 LL and 1 TF doesn't buy you any improvement.

I don't agree : on the P35, 965 anf G965 chipsets running 3 LL + 1 TF, the core doing LL and sharing a die with TF sees a 6% to 12 % improvement over the cores of the die where both are doing LL tests.

Jacob