From: Arnd Bergmann <arnd@arndb.de>
To: =?iso-8859-1?q?J=F6rn_Engel?= <joern@wohnheim.fh-wedel.de>
Subject: Re: [RFC] Alignment of fields in struct dentry
Date: Mon, 18 Sep 2006 23:54:04 +0200
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Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20060914093123.GA10431@wohnheim.fh-wedel.de> <200609152244.07889.arnd@arndb.de> <20060918212423.GB6899@wohnheim.fh-wedel.de>
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On Monday 18 September 2006 23:24, J?rn 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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