Date: Wed, 31 Oct 2012 22:55:42 +0000 From: Attilio Rao <attilio@freebsd.org> To: Andre Oppermann <andre@freebsd.org> Cc: Ian Lepore <freebsd@damnhippie.dyndns.org>, Adrian Chadd <adrian@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org, Peter Jeremy <peter@rulingia.com> Subject: Re: svn commit: r242402 - in head/sys: kern vm Message-ID: <CAJ-FndADYhn6yOEmR91-h0kUVxPZoOm34NwKUGrtYwvaWCXrFQ@mail.gmail.com> In-Reply-To: <50918AAD.2090906@freebsd.org> References: <201210311807.q9VI7IcX000993@svn.freebsd.org> <CAJ-FndDRkBS57e9mzZoJWX5ugJ0KBGxhMSO50KB8Wm8MFudjCA@mail.gmail.com> <1351707964.1120.97.camel@revolution.hippie.lan> <CAJ-FndC7QwpNAjzQTumqTY6Sj_RszXPwc0pbHv2-pRGMqbw0ww@mail.gmail.com> <CAJ-VmokqEFX4wQYh-ojo3kcWUPj5L-V_k0Nj-u3sQByVypkDFw@mail.gmail.com> <CAJ-FndCL7bpkbfaaR%2BaYQAxEBDmgip0QbrE5JhwnbTicSraz9g@mail.gmail.com> <20121031193020.GJ3309@server.rulingia.com> <1351712425.1120.109.camel@revolution.hippie.lan> <50918AAD.2090906@freebsd.org>
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On Wed, Oct 31, 2012 at 8:31 PM, Andre Oppermann <andre@freebsd.org> wrote: > On 31.10.2012 20:40, Ian Lepore wrote: >> >> On Thu, 2012-11-01 at 06:30 +1100, Peter Jeremy wrote: >>> >>> On 2012-Oct-31 18:57:37 +0000, Attilio Rao <attilio@FreeBSD.org> wrote: >>>> >>>> On 10/31/12, Adrian Chadd <adrian@freebsd.org> wrote: >>>>> >>>>> Right, but you didn't make it configurable for us embedded peeps who >>>>> still care about memory usage. >>>> >>>> >>>> How is this possible without breaking the module/kernel ABI? >>> >>> >>> Memory usage may override ABI compatibility in an embedded environment. >>> >>>> All that assuming you can actually prove a real performance loss even >>>> in the new cases. >>> >>> >>> The issue with padding on embedded systems is memory utilisation rather >>> than performance. >>> >> >> There are potential performance hits too, in that embedded systems tend >> to have tiny caches (16K L1 with no L2, that sort of thing), so >> purposely padding things so that large parts of a cache line aren't used >> for anything wastes a scarce resource. > > > You can define CACHE_LINE_SIZE to 0 on those platforms. > Or to make it even more granular there could be a CACHE_LINE_SIZE_LOCKS > that is used for lock padding. I think that this is a bright idea, albeit under the condition that just like CACHE_LINE_SIZE it won't change during STABLE branches timeframe and that it must not be dependent by SMP option. What do you think about this patch?: http://www.freebsd.org/~attilio/cache_line_size_locks.patch Thanks, Attilio -- Peace can only be achieved by understanding - A. Einstein
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