Messages in this thread | | | Date | Wed, 16 Feb 2011 01:11:02 +0100 | From | Michal Soltys <> | Subject | question: cpu.shares and parent-children relationshp in the hierarchy |
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Hi,
I've been testing how this works in practice, and I have few configurations I'm not sure why they behave the way they do.
All the test processes just drain cpu with an infinite loop. All of them are pinned to single cpu core. cgroup with just -o cpu is mounted, and the scenario is following (in brackets - assigned cpu.shares):
root(1024) / \ Y(1024) X(4096) / \ A(8192) B(8192)
Four test processes sit in X, Y, A and B; root is "empty" (effectively idle processes). The one in Y expectedly gets 20% cpu, A and B divide the cpu share equally, but - why do the process in X gets only ~4.75% ? Essentially:
11:36:45 PM PID %usr %system %guest %CPU CPU Command 11:36:50 PM 29472 20.00 0.00 0.00 20.00 1 loadme(y) 11:36:50 PM 29473 4.80 0.00 0.00 4.80 1 loadme(x) 11:36:50 PM 29474 37.60 0.00 0.00 37.60 1 loadme(a) 11:36:50 PM 29475 37.60 0.00 0.00 37.60 1 loadme(b)
In the other words - what is the intended relation between ancestor's cpu.shares and its children ? Looking at the example above, it looks like the task in X should get 4/5 (root unused, 1/5 for Y, the rest for X subtree) * 1/5 (from assigned values in X subtree, 4096/20480) - but that would be ~16%.
The task in X behaves like if X had 1024 - maybe it's always assumed when parent-children relationship is considered, and the actual value is used only when dividing cpu between siblings ?
If I move the test task from X to root, the situation will change to:
11:56:49 PM PID %usr %system %guest %CPU CPU Command 11:56:54 PM 29472 16.60 0.00 0.00 16.60 1 loadme(y) 11:56:54 PM 29473 16.60 0.00 0.00 16.60 1 loadme(root) 11:56:54 PM 29474 33.20 0.20 0.00 33.40 1 loadme(a) 11:56:54 PM 29475 33.40 0.00 0.00 33.40 1 loadme(b)
In this scenario, everything seems as expected - 1/6 for Y, 1/6 for root, 4/6 for X subtree.
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