Date: Fri, 14 Oct 2005 08:58:16 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: B.Candler@pobox.com Cc: max@love2party.net, freebsd-current@freebsd.org, anderson@centtech.com Subject: Re: ufsstat - testers / feedback wanted! Message-ID: <20051014.085816.104604949.imp@bsdimp.com> In-Reply-To: <20051014091004.GC18513@uk.tiscali.com> References: <200510131412.23525.max@love2party.net> <20051013181026.GB27418@odin.ac.hmc.edu> <20051014091004.GC18513@uk.tiscali.com>
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In message: <20051014091004.GC18513@uk.tiscali.com>
Brian Candler <B.Candler@pobox.com> writes:
: On Thu, Oct 13, 2005 at 11:10:26AM -0700, Brooks Davis wrote:
: > > I don't think you can measure one single interger (or 64bit) increase in face
: > > of a operation that has to access backing store. Even if there is a
: > > performance hit, you don't have to build your kernel with the option enabled.
: >
: > The one thing I'd be worried about here is that 64bit updates are
: > expensive on 32bit machines if you want them to be atomic. Relative to
: > backing store they probably still don't matter, but the might be
: > noticable.
:
: I'd be grateful if you could clarify that point for me. Are you saying that
: if I write
:
: long long foo;
: ...
: foo++;
:
: then the C compiler generates code for 'foo++' which is not thread-safe?
: (And therefore I would have to protect it with a mutex or critical section)
:
: Or are you saying that the C compiler inserts its own code around foo++ to
: turn it into a critical section, and therefore runs less efficiently than
: you'd expect?
You have to protect this thread-unsafe operation yourself.
Warner
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