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SubjectRe: [PATCH 0/18] sched: simplified fork, enable load average into LB and power awareness scheduling
On 12/13/2012 07:35 PM, Borislav Petkov wrote:
> On Thu, Dec 13, 2012 at 11:07:43AM +0800, Alex Shi wrote:
>>>> now, on the other hand, if you have two threads of a process that
>>>> share a bunch of data structures, and you'd spread these over 2
>>>> sockets, you end up bouncing data between the two sockets a lot,
>>>> running inefficient --> bad for power.
>>>
>>> Yeah, that should be addressed by the NUMA patches people are
>>> working on right now.
>>
>> Yes, as to balance/powersaving policy, we can tight pack tasks
>> firstly, then NUMA balancing will make memory follow us.
>>
>> BTW, NUMA balancing is more related with page in memory. not LLC.
>
> Sure, let's look at the worst and best cases:
>
> * worst case: you have memory shared by multiple threads on one node
> *and* working set doesn't fit in LLC.
>
> Here, if you pack threads tightly only on one node, you still suffer the
> working set kicking out parts of itself out of LLC.
>
> If you spread threads around, you still cannot avoid the LLC thrashing
> because the LLC of the node containing the shared memory needs to cache
> all those transactions. *In* *addition*, you get the cross-node traffic
> because the shared pages are on the first node.
>
> Major suckage.
>
> Does it matter? I don't know. It can be decided on a case-by-case basis.
> If people care about singlethread perf, they would likely want to spread
> around and buy in the cross-node traffic.
>
> If they care for power, then maybe they don't want to turn on the second
> socket yet.
>
> * the optimal case is where memory follows threads and gets spread
> around such that LLC doesn't get thrashed and cross-node traffic gets
> avoided.
>
> Now, you can think of all those other scenarios in between :-/

You are right. thanks for explanation! :)

Actually, what I went to say is that numa balancing target is pages in
different node memory, but of course, it may improve LLC performance.
>
> Thanks.
>



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