Date: Thu, 20 Feb 2014 14:17:34 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org, "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org> Cc: Adrian Chadd <adrian@freebsd.org>, Alexander Motin <mav@freebsd.org> Subject: Re: [rfc] bind per-cpu timeout threads to each CPU Message-ID: <201402201417.34148.jhb@freebsd.org> In-Reply-To: <201402191602.54465.jhb@freebsd.org> References: <530508B7.7060102@FreeBSD.org> <CAJ-Vmo=KFF_2tdyq1u=jNkWfEe1sR-89t3JNggf7MEvYsF%2BtQg@mail.gmail.com> <201402191602.54465.jhb@freebsd.org>
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On Wednesday, February 19, 2014 4:02:54 pm John Baldwin wrote: > On Wednesday, February 19, 2014 3:04:51 pm 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. > > > > Chasing this stuff down is a pain, because it only really shows up > > when you're doing lots of concurrency. > > > > I'm happy to make this a boot-time option and leave it off for the > > time being. How's that? > > I think having it be a tunable would be good. OTOH, I could also > see another option which would be to pin all clock threads except > for the "default" one by default and only have the option control > whether or not the default thread is pinned to CPU 0 as callers > who use callout_on() are explicitly asking to run the callout on a > specific CPU. (A further variant of this would be to divorce cpu0's swi from the catch-all softclock and let the catch-all softclock float, but bind all the per-cpu swis) -- John Baldwin
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