netdev - Re: [RFC PATCH] dynamic_queue_limit.h: Make the struct ___cacheline_aligned_on

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Message-ID: <1354898530.2707.21.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Fri, 7 Dec 2012 16:42:10 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Joe Perches <joe@...ches.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Tom Herbert <therbert@...gle.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH] dynamic_queue_limit.h: Make the struct
 ___cacheline_aligned_on_smp

On Fri, 2012-12-07 at 08:29 -0800, Joe Perches wrote:
> On Fri, 2012-12-07 at 08:19 -0800, Eric Dumazet wrote:
> > On Fri, 2012-12-07 at 08:05 -0800, Joe Perches wrote:
> > 
> > > So it seemed somewhat sensible to make the
> > > entire struct in a single cacheline.
> > 
> > Any layout change in an object used in network fast path need a complete
> > performance study.
> > 
> > Even if you provide such a study, we'll need to reproduce your numbers
> > here.
> > 
> > BQL/DQL is not on our radars, spending two cache lines on a critical
> > object is fine.
> 
> Well Maybe Tom can provide some information as to why
> the limit variable was cacheline_aligned_in_smp and not
> the struct.
> 
> I didn't find any discussion about it.

Structure alignment has to be at least the maximum of each member's
alignment, so the struct *is* effectively cacheline_aligned_in_smp.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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