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SubjectRe: 2.6.1 and irq balancing


Ethan Weinstein wrote:

> Nakajima, Jun wrote:
>
>>> Admittedly, the machine's load was not high when I took this sample.
>>> However, creating a great deal of load does not change these
>>> statistics at all. Being that there are patches available for 2.4.x
>>> kernels to fix this, I don't think this at all by design, but what
>>> do I know? =)
>>>
>>
>
>> 2.6 kernels don't need a patch to it as far as I understand. Are you
>> saying that with significant amount of load, you did not see any
>> distribution of interrupts? Today's threshold in the kernel is high
>> because we found moving around interrupts frequently rather hurt the
>> cache and thus lower the performance compared to "do nothing". Can you
>> try to create significant load with your network (eth0 and eh1) and see
>> what happens?
>> Jun
>
>
> Here's the situation two days later, I created some brief periods of
> high load on eth1 and I see we have some change:
>
>
> CPU0 CPU1 CPU2 CPU3
> 0: 184932542 0 2592511 0 IO-APIC-edge timer
> 1: 1875 0 0 0 IO-APIC-edge i8042
> 2: 0 0 0 0 XT-PIC cascade
> 3: 3046103 0 0 0 IO-APIC-edge serial
> 8: 2 0 0 0 IO-APIC-edge rtc
> 9: 0 0 0 0 IO-APIC-level acpi
> 14: 76 0 0 0 IO-APIC-edge ide0
> 16: 2978264 0 0 0 IO-APIC-level
> sym53c8xx
> 22: 7838940 0 0 0 IO-APIC-level eth0
> 48: 916078 0 125150 0 IO-APIC-level aic79xx
> 49: 1099375 0 0 0 IO-APIC-level aic79xx
> 54: 51484241 316 50560879 279 IO-APIC-level eth1
> NMI: 0 0 0 0
> LOC: 187530735 187530988 187530981 187530986
> ERR: 0
> MIS: 0
>


Aside from the obvious imbalance between physical CPUs:
I think interrupts should be much more freely balanced between siblings
that share cache, otherwise process a running on CPU0 gets less time than
process b running on CPU1 because of the interrupt load.


>
> My argument is (see below). This is an old 2x pentium2 @400, also
> running 2.6, an old Compaq Proliant to be exact. This machine
> obviously has no HT, so why the balanced load?


IIRC the P2/3 APICs are set to a round robin delivery mode while the P4
ones are not. It is still not ideal though, while you have fairness, you now
have suboptimal performance.


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