Date: Thu, 20 Feb 2014 03:07:19 +0400 From: Slawa Olhovchenkov <slw@zxy.spb.ru> To: Alexander Motin <mav@FreeBSD.org> 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> Subject: Re: [rfc] bind per-cpu timeout threads to each CPU Message-ID: <20140219230719.GM83358@zxy.spb.ru> In-Reply-To: <53052B80.3010505@FreeBSD.org> References: <530508B7.7060102@FreeBSD.org> <CAJ-VmokQ_C=YVpk41_r-QakB46_RWRe0didq1_RrZBMS7hDX-A@mail.gmail.com> <53050D24.3020505@FreeBSD.org> <CAJ-Vmo=KFF_2tdyq1u=jNkWfEe1sR-89t3JNggf7MEvYsF%2BtQg@mail.gmail.com> <53051C71.3050705@FreeBSD.org> <20140219214428.GA53864@zxy.spb.ru> <53052B80.3010505@FreeBSD.org>
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On Thu, Feb 20, 2014 at 12:09:04AM +0200, Alexander Motin 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 <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? > > 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. 100K TCP flows distributed between CPU (100K/N_cpu). every TCP flow several times per seconds touch his callout (*PPS)
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