On 19 February 2014 14:09, Alexander Motin <m...@freebsd.org> wrote:
> On 19.02.2014 23:44, Slawa Olhovchenkov wrote:
>> On Wed, Feb 19, 2014 at 11:04:49PM +0200, Alexander Motin wrote:
>>> On 19.02.2014 22:04, Adrian Chadd wrote:
>>>> On 19 February 2014 11:59, Alexander Motin <m...@freebsd.org> wrote:
>>>>>> So if we're moving towards supporting (among others) a pcbgroup / RSS
>>>>>> hash style work load distribution across CPUs to minimise
>>>>>> per-connection lock contention, we really don't want the scheduler to
>>>>>> decide it can schedule things on other CPUs under enough pressure.
>>>>>> That'll just make things worse.
>>>>> True, though it is also not obvious that putting second thread on CPU
>>>>> run
>>>>> queue is better then executing it right now on another core.
>>>> Well, it depends if you're trying to optimise for "run all runnable
>>>> tasks as quickly as possible" or "run all runnable tasks in contexts
>>>> that minimise lock contention."
>>>> The former sounds great as long as there's no real lock contention
>>>> going on. But as you add more chances for contention (something like
>>>> "100,000 concurrent TCP flows") then you may end up having your TCP
>>>> timer firing stuff interfere with more TXing or RXing on the same
>>>> connection.
>>> 100K TCP flows probably means 100K locks. That means that chance of lock
>>> collision on each of them is effectively zero. More realistic it could
>> What about 100K/N_cpu*PPS timer's queue locks for remove/insert TCP
>> timeouts callbacks?
> I am not sure what this formula means, but yes, per-CPU callout locks can
> much more likely be congested. They are only per-CPU, not per-flow.

It's not just that, but also TX versus RX ACK processing and further
TX being done on different threads.

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