Interactivity degrades with CONFIG_[FAIR]_GROUP_SCHED set in 2.6.24 and later

Interactivity degrades with CONFIG_[FAIR]_GROUP_SCHED set in 2.6.24 and later

[Joe Peterson]

First of all, I am very excited to see the CFS in the kernel.
Interactivity is really great; much better than before.

When going from kernel 2.6.23 to 2.6.24, however, I noticed that
interactivity (namely the smoothness of mouse motion) was noticeably
less smooth under load.  The most obvious case was while pulling down a
Subversion repository; the mouse would freeze for quite long periods
while being moved (i.e. a large fraction of a second), which never
happens under 2.6.23.  I was using ZFS-FUSE on the partition I was
writing to, and it uses a lot of CPU at times, but 2.6.23 seems to
handle it better.

After reading many posts on similar sounding issues, I decided to try
getting later versions of the kernel: 2.6.25-rc5 and the latest 2.6.25
from the git archive (as of yesterday, 3/16) in case recent patches
addressed this.  They both still exibit the issue, however.

I then tried compiling without CONFIG_GROUP_SCHED (which also turned off
CONFIG_FAIR_GROUP_SCHED and a number of other options), and the behavior
seems to be back to the (better) 2.6.23 performance, at least has far as
what I can tell from my tests.

After playing around with the later kernels with CONFIG_GROUP_SCHED
enabled, I noticed a number of random times the mouse was not smooth and
seemed to not be getting an "interactive" amount of CPU time, but I have
a couple of tasks that repeat the problem reliably, so if there is any
debug information I can provide, please let me know, and I'll be glad to
help out in any way I can.

					Thanks, Joe

Re: Interactivity degrades with CONFIG_[FAIR]_GROUP_SCHED set in 2.6.24 and later

[Joe Peterson]

Joe Peterson wrote:
> When going from kernel 2.6.23 to 2.6.24, however, I noticed that
> interactivity (namely the smoothness of mouse motion) was noticeably
> less smooth under load.  The most obvious case was while pulling down a
> Subversion repository; the mouse would freeze for quite long periods
> while being moved (i.e. a large fraction of a second), which never
> happens under 2.6.23.  I was using ZFS-FUSE on the partition I was
> writing to, and it uses a lot of CPU at times, but 2.6.23 seems to
> handle it better.

I just made a couple of debug output files using the default
CONFIG_GROUP_SCHED setting in 2.6.25-rc5-git (from 3/16):

This one was done during a diff between two kernel source dirs:
http://www.skyrush.com/downloads/linux-cfs-debug/cfs-debug-info-2008.03.18-06.37.44

This one was done during an svn checkout:
http://www.skyrush.com/downloads/linux-cfs-debug/cfs-debug-info-2008.03.18-06.38.23

The latter one was done during a period exhibiting what felt like longer
and more profound pauses in mouse movement, however, the first debug
file shows several se.wait_max > 40.  Both cases showed very
choppy/jerky mouse movement with quite obvious freezes when moving the
mouse in circles during the tests.

Here is a snippet of the se.wait_max lines from the first file:

se.wait_max                        :             0.000000
se.wait_max                        :             0.000000
se.wait_max                        :             7.375852
se.wait_max                        :             4.875690
se.wait_max                        :             7.211323
se.wait_max                        :             3.779489
se.wait_max                        :             2.770382
se.wait_max                        :            12.959876
se.wait_max                        :             3.524036
se.wait_max                        :            16.625484
se.wait_max                        :            73.991289
se.wait_max                        :            18.937293
se.wait_max                        :             6.955806
se.wait_max                        :            21.919110
se.wait_max                        :             6.922932
se.wait_max                        :            10.432544
se.wait_max                        :            76.222556
se.wait_max                        :             7.216472
se.wait_max                        :            90.987582
se.wait_max                        :             7.071110
se.wait_max                        :            16.332227
se.wait_max                        :             2.261235
se.wait_max                        :            67.892956
se.wait_max                        :            25.337064
se.wait_max                        :            44.180597
se.wait_max                        :            74.290628
se.wait_max                        :            12.631292
se.wait_max                        :             1.389533
se.wait_max                        :             3.653521
se.wait_max                        :             2.841697
se.wait_max                        :            16.234723
se.wait_max                        :           107.011747
se.wait_max                        :           110.638560
se.wait_max                        :             9.677538
se.wait_max                        :            15.852109
se.wait_max                        :            23.316106
se.wait_max                        :            18.428330
se.wait_max                        :            16.674552
se.wait_max                        :           110.630411
se.wait_max                        :            47.906824
se.wait_max                        :            74.191984
se.wait_max                        :            12.224021
se.wait_max                        :             2.184557
se.wait_max                        :            17.799343
se.wait_max                        :            17.670697
se.wait_max                        :             0.000000
se.wait_max                        :             0.000000

I hope these help; please let me know if I can help further or run more
tests.

						-Thanks, Joe