Date: Thu, 20 Feb 2014 00:09:04 +0200 From: Alexander Motin <mav@FreeBSD.org> To: Slawa Olhovchenkov <slw@zxy.spb.ru> 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: <53052B80.3010505@FreeBSD.org> In-Reply-To: <20140219214428.GA53864@zxy.spb.ru> 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>
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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. -- Alexander Motin
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