linux-kernel - Re: [PATCH] RCU: remove alignment padding from rcu

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Message-ID: <1304684036.1998.10.camel@castor.rsk>
Date:	Fri, 06 May 2011 13:13:56 +0100
From:	Richard Kennedy <richard@....demon.co.uk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH] RCU: remove alignment padding from rcu_data on 64 bit
 builds

On Fri, 2011-05-06 at 08:20 +0200, Eric Dumazet wrote:
...
> We could shrink this structure more, converting some 64bit fields to
> 32bit ones.
> 
> Rationale is the algo is working well on 32bit arches, no need to use
> 64bit fields.
> 
> candidates : completed, gpnum, passed_quiesc_completed, qlen,
> qlen_last_fqs_check, blimit.
> 
> Counters might be converted too.

I don't know the code well enough to make that decision ;)
Is it possible to shrink it enough to free up another cache line ?
(16 longs to ints).

CONFIG_NO_HZ adds 24 bytes so even for users with !CONFIG_NO_HZ the
variables you suggest are not enough to free a cache line.  

regards
Richard


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