Date: Mon, 26 Aug 2013 11:45:28 -0700 From: Davide Italiano <davide@freebsd.org> To: Robert Watson <rwatson@freebsd.org> Cc: FreeBSD Net <freebsd-net@freebsd.org>, Adrian Chadd <adrian@freebsd.org>, freebsd-current <freebsd-current@freebsd.org>, "Alexander V. Chernikov" <melifaro@ipfw.ru> Subject: Re: [rfc] migrate lagg to an rmlock Message-ID: <CACYV=-EdUOe=vMXmX7feT7cAKZoWJF4CkOr=x1OYgPX1yY77Rg@mail.gmail.com> In-Reply-To: <alpine.BSF.2.00.1308241511400.92711@fledge.watson.org> References: <CAJ-Vmo=VKVDEmmPrTbob6Ft%2B7FWypodNoL36Og=7p_CXBSfktg@mail.gmail.com> <5218AA36.1080807@ipfw.ru> <alpine.BSF.2.00.1308241511400.92711@fledge.watson.org>
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On Sat, Aug 24, 2013 at 7:16 AM, Robert Watson <rwatson@freebsd.org> wrote: > On Sat, 24 Aug 2013, Alexander V. Chernikov wrote: > >> On 24.08.2013 00:54, Adrian Chadd wrote: >>> >>> >>> I'd like to commit this to -10. It migrates the if_lagg locking >>> from a rw lock to a rm lock. We see a bit of contention between the >>> transmit and >> >> >> We're running lagg with rmlock on several hundred heavily loaded machines, >> it really works better. However, there should not be any contention between >> receive and transmit side since there is actually no _real_ need to lock RX >> (and even use lagg receive code at all): >> >> http://lists.freebsd.org/pipermail/svn-src-all/2013-April/067570.html > > > We should distinguish "lock contention" from "line contention". When > acquiring a rwlock on multiple CPUs concurrently, the cache lines used to > implement the lock are contended, as they must bounce between caches via the > cache coherence protocol, also referred to as "contention". In the if_lagg > code, I assume that the read-only acquire of the rwlock (and perhaps now > rmlock) is for data stability rather than mutual exclusion -- e.g., to allow > processing to completion against a stable version of the lagg configuration. > As such, indeed, there should be no lock contention unless a configuration > update takes place, and any line contention is a property of the locking > primitive rather than data model. > > There are a number of other places in the kernel where migration to an > rmlock makes sense -- however, some care must be taken for four reasons: (1) > while read locks don't experience line contention, write locking becomes > observably e.g., rmlocks might not be suitable for tcbinfo; (2) rmlocks, > unlike rwlocks, more expensive so is not suitable for all rwlock line > contention spots -- implement reader priority propagation, so you must > reason about; and (3) historically, rmlocks have not fully implemented > WITNESS so you may get less good debugging output. if_lagg is a nice place I'm not sure what you mean here with (3), because from my understanding of the code WITNESS is implemented both in the sleepable and non-sleepable case, but there could be something I'm missing. Something I think we lack in rmlock code is fully supported LOCK_PROFILING as we have in all the other primitives, but again, if I'm wrong feel free to correct me. > to use rmlocks, as reconfigurations are very rare, and it's really all about > long-term data stability. > > Robert > > _______________________________________________ > freebsd-current@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to "freebsd-current-unsubscribe@freebsd.org" Thanks, -- Davide "There are no solved problems; there are only problems that are more or less solved" -- Henri Poincare
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