linux-kernel - Re: [RFC] Alignment of fields in struct dentry

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Message-Id: <200609182354.04781.arnd@arndb.de>
Date:	Mon, 18 Sep 2006 23:54:04 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Jörn Engel <joern@...nheim.fh-wedel.de>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] Alignment of fields in struct dentry

On Monday 18 September 2006 23:24, Jörn Engel wrote:
> On Fri, 15 September 2006 22:44:07 +0200, Arnd Bergmann wrote:
> >
> > I'd guess that a 32 byte alignment is much better here, 64 byte sounds
> > excessive. It should have the same effect with the current dentry layout
> > and default config options, but would keep the d_iname length in the
> > 16-44 byte range instead of 16-76 byte as your patch does.
> > 
> > Since all important fields are supposed to be kept in 32 bytes anyway,
> > they are still either at the start or the end of a given cache line,
> > but never cross two.
> 
> Another take would be to use a cacheline.  But I guess the difference
> between 32/64/cacheline is mostly academic, given the rate of changes
> to struct dentry.

There have been so many optimizations and misoptimizations regarding
the dentry struct over the years. See http://lkml.org/lkml/2004/5/8/117
for the almost exact opposite of this patch, along with the same discussion
that we're having now.

	Arnd <><
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