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Old 2008-04-02, 18:02   #1
mdettweiler
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Default Prime-search apps run slowly on Ubuntu 8.04

Hi all,

I've noticed that ever since I upgraded to Ubuntu 8.04 beta (from 7.10) last night, both sr2sieve and LLR (the two prime-search apps I'm running at the time) are running at about half their previous speed. For example, whereas before I got more than a million p/sec with sr2sieve on a given sieve, now I'm getting about 668,000 p/sec.

I was wondering if some of this might have to do with the scheduler "improvements" added to the Linux kernel that are being included in 8.04, so I booted by system using the slightly older kernel available on the GRUB menu (which, as far as I know, doesn't have the scheduler "improvements" in it)--and, oddly enough, it didn't fix the problem.

Has anyone else seen this? I at first thought that it might be dust in the CPU fan, but then I noticed that the slowup coincided exactly with my Ubuntu upgrade, so my hunch is that it's tied to that.

Last fiddled with by mdettweiler on 2008-04-02 at 18:03 Reason: typo
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Old 2008-04-02, 19:19   #2
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My guess is that you're running the jobs at less than full priority, and Ubuntu runs the CPU at minimum clock if only lower-priority jobs are running.

Put some CPU Frequency Scaling Monitors in your panel, and set the mode to 'performance', and your CPU will run at full clock 24/7
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Old 2008-04-02, 19:45   #3
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Quote:
Originally Posted by fivemack View Post
My guess is that you're running the jobs at less than full priority, and Ubuntu runs the CPU at minimum clock if only lower-priority jobs are running.

Put some CPU Frequency Scaling Monitors in your panel, and set the mode to 'performance', and your CPU will run at full clock 24/7
Ta-daa! That's it! Yep, the applet on the panel says 1.20 Ghz (it should be 2.2Ghz).

One quick question, though: where is the setting that I need to change so that it runs in "Performance" mode?
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Old 2008-04-02, 20:00   #4
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Quote:
Originally Posted by Anonymous View Post
Ta-daa! That's it! Yep, the applet on the panel says 1.20 Ghz (it should be 2.2Ghz).

One quick question, though: where is the setting that I need to change so that it runs in "Performance" mode?
Well, it turns out a quick Google search got me the answer to that one. I apparently needed to run "sudo dpkg-reconfigure gnome-applets" so I could tell it to allow regular users to be able to change the CPU scaling options from the panel applet.

Thanks, fivemack, for pointing that out to me! Now both sr2sieve and LLR are operating at more "normal" rates.
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Old 2008-04-03, 10:15   #5
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Should this advice be added to readme.txt, in a Linux section?
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Old 2008-04-03, 18:24   #6
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Quote:
Originally Posted by cheesehead View Post
Should this advice be added to readme.txt, in a Linux section?
Well, I don't know if it has to do with any other distributions than Ubuntu 8.04--and remember, 8.04 is still in beta, so this might very well be fixed in time for the release. So, it might not yet warrant being put into readme.txt.
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Old 2008-04-03, 21:07   #7
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Quote:
Originally Posted by Anonymous View Post
Well, I don't know if it has to do with any other distributions than Ubuntu 8.04--
I meant to include a conditional suggestion:

If after an OS upgrade you find that your prime-search apps are running markedly slower, try the following steps that may help:

1. ~~~

2. ~~~
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Old 2008-04-04, 01:50   #8
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Quote:
Originally Posted by cheesehead View Post
I meant to include a conditional suggestion:

If after an OS upgrade you find that your prime-search apps are running markedly slower, try the following steps that may help:

1. ~~~

2. ~~~
Ah, I see. Yeah, that would probably be a good idea to put in the readme.txt file, then.
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Old 2008-04-04, 03:01   #9
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This `turn down the CPU speed when all active programs are running at low priority' feature will probably find its way into other operating systems too.

Does anyone know the minimum priority level a program can run at without this feature being activated?
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Old 2008-04-04, 03:34   #10
mdettweiler
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Quote:
Originally Posted by geoff View Post
This `turn down the CPU speed when all active programs are running at low priority' feature will probably find its way into other operating systems too.

Does anyone know the minimum priority level a program can run at without this feature being activated?
Well, I even tried running the programs at a nice level of 2 (i.e. the lower end of "normal" range) and the OS still didn't kick the CPU into high gear. Maybe this is a fault of Ubuntu 8.04, though--it is still in beta, after all. Earlier today, though, I downloaded and installed a bunch of updates to the beta distribution--I still need to reboot my system for them to take effect. When I've done that, though, I'll be sure to check if the problem was fixed.

Last fiddled with by mdettweiler on 2008-04-04 at 03:35
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Old 2008-04-04, 16:58   #11
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After the reboot, I still found that my CPU refused to budge from 1.2Ghz at the behest of prime-searching apps, thus I had to adjust the priority using the GNOME panel applet that fivemack put me wise to.

However, a new update for "linux-ubuntu-modules-2.6.24-generic" just showed up in Update Manager this morning, maybe that will fix it. The updates (56 of them! Yeah, things change quickly in beta. ) are currently being downloaded.
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