Date: Thu, 29 Aug 2013 12:01:03 -0400 From: John Baldwin <jhb@freebsd.org> To: Scott Long <scott4long@yahoo.com> Cc: FreeBSD Net <freebsd-net@freebsd.org>, Adrian Chadd <adrian@freebsd.org>, freebsd-current@freebsd.org, Robert Watson <rwatson@freebsd.org>, "Alexander V. Chernikov" <melifaro@ipfw.ru> Subject: Re: [rfc] migrate lagg to an rmlock Message-ID: <201308291201.04131.jhb@freebsd.org> In-Reply-To: <41614148-3900-4FE0-88AC-40F10DAE2030@yahoo.com> References: <CAJ-Vmo=VKVDEmmPrTbob6Ft%2B7FWypodNoL36Og=7p_CXBSfktg@mail.gmail.com> <201308291042.13282.jhb@freebsd.org> <41614148-3900-4FE0-88AC-40F10DAE2030@yahoo.com>
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On Thursday, August 29, 2013 11:37:08 am Scott Long wrote: > > On Aug 29, 2013, at 7:42 AM, John Baldwin <jhb@freebsd.org> wrote: > > > On Saturday, August 24, 2013 10:16:33 am Robert Watson wrote: > >> 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 to use rmlocks, as > >> reconfigurations are very rare, and it's really all about long-term data > >> stability. > > > > 3) should no longer be an issue. rmlocks now have full WITNESS and assertion > > support (including an rm_assert). > > > > However, one thing to consider is that rmlocks pin readers to CPUs while the > > read lock is held (which rwlocks do not do). > > And this is not a problem for the application that we're giving it in the > lagg driver. That is likely true. I was merely tweaking Robert's general guidelines re: rmlock. -- John Baldwin
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