Date: Mon, 5 Nov 2012 10:11:52 -0700 From: Warner Losh <imp@bsdimp.com> To: Eitan Adler <lists@eitanadler.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: <B4225C25-BD43-423C-A1A2-C9FD4AC92ECB@bsdimp.com> In-Reply-To: <CAF6rxg=Et1d6u4RBCB88KibW_uiaRbNdb75v0TQOr-0BrEXV=g@mail.gmail.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>
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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. >>=20 >> Is that an out-of-kernel interface? >>=20 >> 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. >=20 > 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). >=20 > 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: >=20 > For more details see > http://comments.gmane.org/gmane.os.freebsd.devel.cvs/483696 > which contains the original discussion. >=20 > 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
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