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SubjectRe: [PATCH][IO_APIC] 2.5.63bk7 irq_balance improvments / bug-fixes
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Hi Andrew, Kai,

The bouncing is seen because of the round robin IRQ distribution in
some
particular cases. In some cases, (such as single heavy interrupt source
in
a 2way SMP system) binding heavy interrupt sources to different cpus is
not
going to remove the complete imbalance. In that case we fall back to
Ingo's
round robin approach. We have studied the previous round robin interrupt

distribution implemented in the kernel, and we found that, at very high
interrupt rate, the performance of the system increased with the
increasing
period of the round robin distribution. Please see the original LKML
posting
for more details.
http://www.uwsg.indiana.edu/hypermail/linux/kernel/0212.2/1122.html

So when if there is significant imbalance left after binding the IRQs to
cpus,
there are two options now,

1. Do not move around. Let the significant imbalance stick on a
particular
cpu.

2. Or move the heavy imbalance around all the cpus in the round robin
fashion at high rate.

Also we can have either of the option configurable in the kernel.

Both the solutions will eliminate the bouncing behavior. The current
implementation is based on the option 2, with the only difference of
lower rate of distribution (5 sec). The optimal option is workload
dependant. With static and heavy interrupt load, the option 2 looks
better, while with random interrupt load the option 1 is good enough.

Thanks & Regards,
Nitin


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