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Subject[PATCH 0/4] sched: Add CPU rate caps
These patches implement CPU usage rate limits for tasks.

Although the rlimit mechanism already has a CPU usage limit (RLIMIT_CPU)
it is a total usage limit and therefore (to my mind) not very useful.
These patches provide an alternative whereby the (recent) average CPU
usage rate of a task can be limited to a (per task) specified proportion
of a single CPU's capacity. The limits are specified in parts per
thousand and come in two varieties -- hard and soft. The difference
between the two is that the system tries to enforce hard caps regardless
of the other demand for CPU resources but allows soft caps to be
exceeded if there are spare CPU resources available. By default, tasks
will have both caps set to 1000 (i.e. no limit) but newly forked tasks
will inherit any caps that have been imposed on their parent from the
parent. The mimimim soft cap allowed is 0 (which effectively puts the
task in the background) and the minimim hard cap allowed is 1.

Care has been taken to minimize the overhead inflicted on tasks that
have no caps and my tests using kernbench indicate that it is hidden in
the noise.


1. Caps are not enforced for SCHED_FIFO and SCHED_RR tasks.

2. This versions incorporates improvements and bug fixes as a result of
feedback from an earlier post. Special thanks to Con Kolivas whose
suggestions with respect to improved methods for avoiding starvation
and priority inversion have enabled cap enforcement to be stricter.

3. This patch is against 2.6.17-rc6-mm2.

4. Overhead Measurements. To measure the implications for overhead
introduced by these patches kernbench was used on a dual 500Mhz
Centrino SMP system. Runs were done for a kernel without these
patches applied, one with the patches applied but no caps being used
and one with the patches applied and running kernbench with a soft cap
of zero (which would be inherited by all its children).

Average Optimal -j 8 Load Run:

Vanilla Patch Applied Soft Cap 0%

Elapsed Time 1056.1 (1.92) 1048.2 (0.62) 1064.1 (1.59)
User Time 1908.1 (1.09) 1895.2 (1.30) 1926.6 (1.39)
System Time 181.7 (0.60) 177.5 (0.74) 173.8 (1.07)
Total 2089.8 2072.7 2100.4
Percent CPU 197.6 (0.55) 197.0 (0) 197.0 (0)
Context Switches 49253.6 (136.31) 48881.4 (92.03) 92490.8 (163.71)
Sleeps 28038.8 (228.11) 28136.0 (250.65) 25769.4 (280.40)

As can be seen there is no significant overhead penalty for having
these patches applied and leaving them unused (in fact, based on
these numbers, there's actually a small improvement). Similarly,
the overhead for running kernbench as a background (soft cap of
zero) job is not significant. As expected, the context switches for
the background run are double (due to the fact that ANY OTHER task
running on the machine would be able to preempt the kernbench tasks)
but have not seriously effected the CPU usage statistics. The
similarity of the "Percent CPU" numbers indicate that load balancing
hasn't been adversely effected.

5. Code size measurements:

Vanilla kernel:

text data bss dec hex filename
33800 4689 296 38785 9781 sched.o
2554 79 0 2633 a49 mutex.o
12076 2632 0 14708 3974 base.o

Patches applied:

text data bss dec hex filename
36870 4721 296 41887 a39f sched.o
2630 79 0 2709 a95 mutex.o
13011 2920 0 15931 3e3b base.o

Indicating that the size cost of the patch proper is about
3 kilobytes and the procfs costs about another 1.2 kilobytes.

Signed-off-by: Peter Williams <>

Peter Williams

"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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