Re: [rfc] bind per-cpu timeout threads to each CPU

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 <mav_at_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.

-- 
Alexander Motin