linux-kernel - 2.6.22.2-cfs-v19.1 stops honouring nice 3 after some hours

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Date:	Wed, 15 Aug 2007 18:23:17 +0200
From:	Michael Gerdau <mgd@...hnosis.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org
Subject: 2.6.22.2-cfs-v19.1 stops honouring nice 3 after some hours

Hi Ingo,

I played a bit with renicing my (mostly) CPU bound jobs and found
somewhat strange behaviour.

I'm on 2.6.22.2-cfs-v19.1 -- I can't boot into 2.6.23-rc3 as part of
my daily work because my NVIDIA driver doesn't yet work with it though
I could try to do some X-less testing if need be.

I'm running 4 CPU bound jobs, 2 of which are more important to me.
I therefor reniced the other 2 to level 3 and the CPU percentages
were roughly 66/66/33/33 on my Dualcore system. So far all fine.

Some 6 hours later I rechecked with top -d 60 and got the output below:

19629 mgd       20   0  334m 263m 2700 R   49  8.1  69:16.45 perl
29668 mgd       20   0  396m 262m 2700 R   49  8.0 112:17.21 perl
 6933 mgd       23   3 96576  21m 3672 R   47  0.7  46:38.83 perl
14162 mgd       23   3  165m  28m 3672 R   47  0.9 194:55.31 perl

I reniced to 0:

19629 mgd       20   0  334m 263m 2700 R   48  8.1  71:02.86 perl
 6933 mgd       20   0 96652  21m 3672 R   48  0.7  48:23.41 perl
14162 mgd       20   0  165m  28m 3676 R   48  0.9 196:39.81 perl
29668 mgd       20   0  396m 262m 2700 R   48  8.0 114:03.56 perl

...and back to 3 with no noticeable change.

[at this point job 14162 finished and was replaced by an similar
job 22558]

I then decided to do some statistics and reniced to 1:

29668 mgd       20   0  396m 262m 2700 R   48  8.0 115:24.12 perl
19629 mgd       20   0  334m 263m 2700 R   48  8.1  72:23.36 perl
22558 mgd       21   1 92184  17m 3672 R   48  0.5   0:56.81 perl
 6933 mgd       21   1 96576  21m 3672 R   48  0.7  49:54.12 perl

Now I reniced to 2 and without knowing why, the ratios seem to be back
to where I want them.

29668 mgd       20   0  396m 262m 2700 R   59  8.0 116:33.98 perl
19629 mgd       20   0  334m 264m 2700 R   58  8.1  73:31.49 perl
22558 mgd       22   2 92220  17m 3672 R   37  0.5   1:46.99 perl
 6933 mgd       22   2 96688  21m 3672 R   36  0.7  50:44.77 perl

Renicing to 3 again gives me what I want:

29668 mgd       20   0  397m 262m 2700 R   65  8.0 117:25.93 perl
19629 mgd       20   0  334m 264m 2700 R   63  8.1  74:21.84 perl
22558 mgd       23   3 92220  17m 3672 R   32  0.5   2:13.83 perl
 6933 mgd       23   3 96652  21m 3672 R   32  0.7  51:11.24 perl


I don't know why I had the very first output above though.

Best,
Michael
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