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

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Date:	Fri, 07 Dec 2012 08:54:29 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	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:

> 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.
> 

The struct _is_ cache line aligned, since it contains one field needing
cache line alignment. Its pretty clear to us.

There are two cache lines in it.

We don't one a single cache line, but two, for performance reasons.

If you believe its wrong, you have to provide the performance study, not
me, as I don't have time to spend cycles on this. We did this a long
time ago.

If you want to save few bytes on your kernel, redefine
__cacheline_aligned_on_smp to empty, and you'll be ok.

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