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SubjectRe: [RFC] scheduler: improve SMP fairness in CFS

* Tong Li <tong.n.li@intel.com> wrote:

> This patch extends CFS to achieve better fairness for SMPs. For
> example, with 10 tasks (same priority) on 8 CPUs, it enables each task
> to receive equal CPU time (80%). [...]

hm, CFS should already offer reasonable long-term SMP fairness. It
certainly works on a dual-core box, i just started 3 tasks of the same
priority on 2 CPUs, and on vanilla 2.6.23-rc1 the distribution is this:

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7084 mingo 20 0 1576 248 196 R 67 0.0 0:50.13 loop
7083 mingo 20 0 1576 244 196 R 66 0.0 0:48.86 loop
7085 mingo 20 0 1576 244 196 R 66 0.0 0:49.45 loop

so each task gets a perfect 66% of CPU time.

prior CFS, we indeed did a 50%/50%/100% split - so for example on
v2.6.22:

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2256 mingo 25 0 1580 248 196 R 100 0.0 1:03.19 loop
2255 mingo 25 0 1580 248 196 R 50 0.0 0:31.79 loop
2257 mingo 25 0 1580 248 196 R 50 0.0 0:31.69 loop

but CFS has changed that behavior.

I'll check your 10-tasks-on-8-cpus example on an 8-way box too, maybe we
regressed somewhere ...

Ingo
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