Date: Mon, 05 Nov 2012 10:38:07 -0700 From: Ian Lepore <freebsd@damnhippie.dyndns.org> To: Warner Losh <imp@bsdimp.com> Cc: "Rodney W. Grimes" <freebsd@pdx.rh.cn85.chatusa.com>, Juli Mallett <juli@clockworksquid.com>, "freebsd-mips@FreeBSD.org" <freebsd-mips@freebsd.org> Subject: Re: CACHE_LINE_SIZE macro. Message-ID: <1352137087.1120.180.camel@revolution.hippie.lan> In-Reply-To: <B4225C25-BD43-423C-A1A2-C9FD4AC92ECB@bsdimp.com> References: <CACVs6=_BrwJ19CPj7OqKvV8boHfujVWqn96u3VPUmZ040JpAeQ@mail.gmail.com> <201211041828.qA4ISomC076058@pdx.rh.CN85.ChatUSA.com> <CAF6rxgn-bNJOuvdiRj_UUGQUTRaeOt54OdzHOioNz5f566hoig@mail.gmail.com> <DAE462F0-9D85-4942-8826-C0709E36D3B7@bsdimp.com> <CAF6rxg=Et1d6u4RBCB88KibW_uiaRbNdb75v0TQOr-0BrEXV=g@mail.gmail.com> <B4225C25-BD43-423C-A1A2-C9FD4AC92ECB@bsdimp.com>
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On Mon, 2012-11-05 at 10:11 -0700, Warner Losh wrote: > On Nov 5, 2012, at 10:01 AM, Eitan Adler wrote: > > > On 5 November 2012 11:49, Warner Losh <imp@bsdimp.com> wrote: > >>> There has been some discussion recently about padding lock mutexs to > >>> the cache line size in order to avoid false sharing of CPUs. Some have > >>> claimed to see significant performance increases as a result. > >> > >> Is that an out-of-kernel interface? > >> > >> If we did that, we'd have to make it run-time settable, because there's no one right answer for arm and MIPS cpus: they are all different. > > > > The discussion ended up with using a special parameter > > CACHE_LINE_SIZE_LOCKS which is different than CACHE_LINE_SIZE. This is > > necessary for other reasons as well (CACHE_LINE_SIZE_LOCKS may take > > into account prefetching of cache lines, but CACHE_LINE_SIZE > > wouldn't). > > > > I think the "correct" thing to do here is choose a reasonable, but > > not-always-correct CACHE_LINE_SIZE_LOCKS and make CACHE_LINE_SIZE a > > per-board constant (or run time setting, or whatever works). You > > can't make it run-time settable as the padding is part of the ABI: > > > > For more details see > > http://comments.gmane.org/gmane.os.freebsd.devel.cvs/483696 > > which contains the original discussion. > > > > Note - I was not involved. > > this is a kernel-only interface, so compile time constants are fine there. What user-land visible interfaces are affected by this setting? The answer should be 'none' > > Warner When I commented on Attilio's recent checkins concerning padding of locks to cache line size and the fact that the value changes per-cpu and we're not well-positioned to handle that right now, his main concern was modules matching the kernel. I had suggested making the padding conditional on SMP (because apparently there's no benefit to the padding in a UP kernel), but then a module compiled for UP wouldn't work right on an SMP kernel, and vice versa. I'm not sure why that's a problem, my solution to that would be "So then don't do that." What scares me the most is the mushy definition of what CACHE_LINE_SIZE really means. There's nothing about the name that says "This may not be the actual cache line size but it's probably close," but increasingly I see people talking about it as if it had such a malleable meaning. Is that consistant with the existing uses in the code? Is it a good idea? -- Ian
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