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Old 2010-08-02, 05:56   #8
A Sunny Moo
mdettweiler's Avatar
Aug 2007

11000011010012 Posts

Originally Posted by Rodrigo View Post

Fascinating -- thank you! This is growing on me fast.

You're right, the second exponent IS a little lower than the first one. And after a little less than one hour doing P-1 work on it, almost 4% of stage 1 is complete. So I guess that it's not necessary to increase the RAM setting. (Right?)
I would HIGHLY recommend increasing the RAM. Even if you're going to be doing primarily LL work, the occasional P-1 pretesting stint is going to need a lot more than 8 MB to do a decent job. Adding more memory is not going to speed it up, but what it will do is increase your chances of finding a factor in P-1 (and therefore eliminating the need for the LL test) by quite a bit. What Prime95 does is automatically calculate how much time it should optimally spend on P-1 relative to the LL test. At a given set of bounds (B1 and B2) P-1 will run faster with more memory. Thus, as you add more memory, Prime95 increases the bounds to compensate--therefore taking the same amount of time but increasing the chance of a factor.

As George suggested earlier, if you can spare it, 500MB is a good minimum amount to allot for P-1. Beyond that point adding more still helps, though it's around there that the returns begin to diminish rather quickly. Note that the full RAM allotment is only used during P-1 stage 2--everything else (including the LL test itself and any TF) uses a minimal amount of RAM (a few MB) because it doesn't need any more than that. So if having 500MB less RAM for a few days at the beginning of some LL tests is not going to severely impact performance on your computer, I'd definitely go for it.

The "Per iteration time" on the first worker increased from 0.068 seconds to 0.079 seconds after I got the second worker going. Is that a coincidence, or a result of starting the second worker?
My guess is that the hit is due to the slight CPU overhead created by normal use of the computer (if you had a web browser open with a few tabs or maybe a webmail interface, that would do it). When you only have one worker going, the other core is completely idle--which means that the operating system's scheduler puts any additional requests for CPU time (web browser, etc.) onto the idle core, leaving the worker all to itself on one core. But when you start the second worker, both cores are filled, which means that the scheduler has got to arbitrarily choose which core to send additional time requests to--which can mean one core or the other at any given moment. The unlucky core then takes a bit of a performance hit on Prime95.

Disclaimer: this is just a hypothesis. I could be completely off-base.

And finally (for now!), if only Prime95 is running on the PC (such that it's almost always at 100%), does that help the computer to finish the exponent faster?
Marginally. As I explained above, normal use of the computer does take away a little bit from Prime95. However, as long as you don't have any programs running that drain much CPU time over the long term (most programs don't), you shouldn't see much of a difference.
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