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Date: Thu, 20 Feb 2014 01:44:28 +0400
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Alexander Motin <mav@FreeBSD.org>
Subject: Re: [rfc] bind per-cpu timeout threads to each CPU
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Cc: Adrian Chadd <adrian@freebsd.org>,
 freebsd-current <freebsd-current@freebsd.org>,
 Jeffrey Faden <jeffreyatw@gmail.com>,
 "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org>
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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@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?