Comment 0 for bug 1219548

With the default Ubuntu 13.04 sysctl settings and boot parameters, positively niced tasks are very clearly given a larger share of CPU time than they ought to be. The update-apt-xapian-index process, for instance, runs at niceness +19; but it still slows my Celeron M520 laptop to a crawl. Likewise for other positively niced tasks.

If, on the other hand, I boot with the 'noautogroup' kernel parameter (which sets kernel.sched_autogroup_enabled to 0), responsiveness is much better when running CPU heavy tasks at positive niceness.

In both cases, processes are reported by top and ps as running at the correct niceness; it appears that the nice level is ignored silently.

Autogrouping is supposed to provide better responsiveness under some loads, but I believe that breaking nice settings is too high a price. I think it would be a good idea to turn off kernel.sched_autogroup_enabled, until this behavior is fixed upstream.

By the way, an important note: modifying the sched_autogroup_enabled sysctl on a running system will usually cause a kernel panic. Autogrouping needs to be turned off using 'noautogroup' as a boot parameter; if you try it using sysctl.conf you will get a nice kernel panic on boot.