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Date:      Wed, 31 Oct 2012 13:40:25 -0600
From:      Ian Lepore <freebsd@damnhippie.dyndns.org>
To:        Peter Jeremy <peter@rulingia.com>
Cc:        Attilio Rao <attilio@freebsd.org>, svn-src-head@freebsd.org, Adrian Chadd <adrian@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org
Subject:   Re: svn commit: r242402 - in head/sys: kern vm
Message-ID:  <1351712425.1120.109.camel@revolution.hippie.lan>
In-Reply-To: <20121031193020.GJ3309@server.rulingia.com>
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>

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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.

That said, I think a point Attilio was trying to make is that we won't
see a large hit because this doesn't affect a large number of mutex
instances.  I'm willing to accept his expert advice on that, not in
small part because I'm not sure how I'd go about disputing it. :)

I'm really busy with $work right now, but things should calm down in a
couple weeks, and I'd be willing to do some measurements on arm systems
then, if I can get some help on how to generate useful data.

-- Ian





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