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Old 2005-09-07, 21:33   #1
Jushi
 
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Sep 2005
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Default Wildly differing times for step 2 with ecm-6.0.1

I recently started exploring GMP-ECM and noted that for a given number and given parameters, the Step 1 time is almost constant, but Step 2 time varies from run to run up to a factor 5 or so. This is on one and the same machine for 5 curves:

Code:
Step 1 took 1188420ms
Step 1 took 1188460ms
Step 1 took 1189800ms
Step 1 took 1188310ms
Step 1 took 1189760ms
Code:
Step 2 took 671880ms
Step 2 took 718610ms
Step 2 took 2851240ms
Step 2 took 475260ms
Step 2 took 558580ms
Is this normal or am I doing something wrong?
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Old 2005-09-07, 21:38   #2
akruppa
 
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Stage 2 can use quite a lot of RAM. Is the computer running out of memory and starting to swap to disk a lot? In that case, the performance you'd get would depend i.e. on how many other applications are running.

What system are you on, what number are you factoring and with what parameters?

Alex
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Old 2005-09-10, 08:11   #3
Jushi
 
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Looking at a `top', I think I can explain the problem: a load average of 1.3 (should be 1.0). However, apart from `ecm' I see nothing else running... so that will require some investigation.

By the way: the machine is a PentiumIV 3.0GHz with 2GB ram and zero swap. The number is a C288, and I'm running ecm-6.0.1 with B1=11e6.

Other question: Is there any difference between running `ecm' ten times, as opposed to running once with a -c 10 option?
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Old 2005-09-10, 09:23   #4
geoff
 
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Quote:
Originally Posted by Jushi
By the way: the machine is a PentiumIV 3.0GHz with 2GB ram and zero swap. The number is a C288, and I'm running ecm-6.0.1 with B1=11e6.
Those stage one times (and the fastest stage two time) are almost double what a number that size should require on that machine.
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Old 2005-09-11, 21:25   #5
Jushi
 
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The problem with differing times seems to have resolved itself by rebooting (I still don't know what caused it).

Quote:
Originally Posted by geoff
Those stage one times (and the fastest stage two time) are almost double what a number that size should require on that machine.
Are you sure? What else could influence the running time...?
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Old 2005-09-11, 23:19   #6
Mystwalker
 
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Quote:
Originally Posted by Jushi
Are you sure? What else could influence the running time...?
Maybe another application that takes "full" processing power, in case you have Hyperthreading enabled?
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Old 2005-09-12, 00:33   #7
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Quote:
Originally Posted by Mystwalker
Maybe another application that takes "full" processing power, in case you have Hyperthreading enabled?
Another possibility is that you are using a GMP library optimised for the wrong processor. If that is the case then recompiling with P4-specific optimisations could give a very large improvement. (For example, the precompiled Debian library is optimised for a 486, recompiling for a P4 makes ecm run more than 70% faster).
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Old 2005-09-12, 01:30   #8
geoff
 
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As a reference point, here is a curve for a general 284 digit number on a 2.66GHz P4:

GMP-ECM 6.0 [powered by GMP 4.1.4] [ECM]
Input number is 1659 ... 8037 (284 digits)
Using B1=11000000, B2=25577181640, polynomial Dickson(12), sigma=3150013712
Step 1 took 645928ms
Step 2 took 208735ms
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