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Date: Fri, 14 Oct 2005 10:10:04 +0100
From: Brian Candler <B.Candler@pobox.com>
To: Brooks Davis <brooks@one-eyed-alien.net>
Message-ID: <20051014091004.GC18513@uk.tiscali.com>
References: <434E46C0.7060903@centtech.com>
	<200510131412.23525.max@love2party.net>
	<20051013181026.GB27418@odin.ac.hmc.edu>
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Cc: Max Laier <max@love2party.net>, freebsd-current@freebsd.org,
	Eric Anderson <anderson@centtech.com>
Subject: Re: ufsstat - testers / feedback wanted!
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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?

Regards,

Brian.