To: "Julio M. Merino Vidal" <jmerino@ac.upc.edu>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Per-CPU data as a structure
References: <16C35E62-A26F-49FC-9CD8-7BADFCD97BA5@ac.upc.edu>
From: Andi Kleen <andi@firstfloor.org>
Date: 03 May 2007 19:19:21 +0200
In-Reply-To: <16C35E62-A26F-49FC-9CD8-7BADFCD97BA5@ac.upc.edu>
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"Julio M. Merino Vidal" <jmerino@ac.upc.edu> writes:
 
> Similarly, and if I understood it correctly, the PDA (Per-processor
> Data Area) also aims to do the above, but at the moment it only
> contains some fields and is not defined in all platforms.  There are
> still a lot of usages of the percpu functionality (such as, e.g., in
> kernel/sched.c).

PDA is an earlier version of percpu; it still can be more efficiently
accessed so it is kept for some low level code.
 
> As far as I can tell, the advantage of percpu is that you can define
> new "fields" anywhere in the code and independently from the rest of
> the system. 

- Independent maintenance as you noted
- Fast access and relatively compact code
- Avoids false sharing by keeping cache lines of different CPUs separate
- Doesn't waste a lot of memory in padding like NR_CPUs arrays usually
need to to avoid the previous point.

Any replacement that doesn't have these properties too will probably
be not useful.

-Andi

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