linux-kernel - Re: [PATCH 1/2] Define new percpu interface for shared data -- version 3

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Message-Id: <20070523123153.c4f8c7e6.akpm@linux-foundation.org>
Date:	Wed, 23 May 2007 12:31:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ravikiran G Thirumalai <kiran@...lex86.org>
Cc:	"Yu, Fenghua" <fenghua.yu@...el.com>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>, clameter@....com,
	rmk@....linux.org.uk, linux-kernel@...r.kernel.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com
Subject: Re: [PATCH 1/2] Define new percpu interface for shared data --
 version 3

On Wed, 23 May 2007 12:20:05 -0700 Ravikiran G Thirumalai <kiran@...lex86.org> wrote:

> On Wed, May 23, 2007 at 12:09:56PM -0700, Yu, Fenghua wrote:
> > 
> > >Has there been any measurable benefit yet due to tail padding?
> > 
> > We don't have data that tail padding actually helps. It all
> > depends on what data the linker lays out in the cachelines.
> > 
> > As of now we just want to create the infrastructure (so that
> > more and more people who need it, can use it).
> 
> So what we have now is space wastage on some architectures, space savings on
> some, but with no measurable performance benefit due to the infrastructure
> itself.  Why not push the infrastructure when we really need it, as against
> pushing it now when we are not sure if it benefits?
> 

It makes sense from a theoretical POV and is pretty much a no-op in terms
of resource consumption.

The problem with the wait-until-it-hurts approach is that by the time
someone hurts from this and we find out about it, they may well be using
some year-old enterprise kernel and it's too late to fix it for them.

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