lkml.org 
[lkml]   [2007]   [May]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: Per-CPU data as a structure
Andi Kleen wrote:
>> 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.
>
Thank you for the details. I'll try to stick to per-cpu wherever
possible for now.

Anyway, what do you think about adding the above text to the code (percpu.h
maybe) as documentation? See the patch below. (Dunno if the Signed-off-by
line is appropriate as most of the text is yours.)

Signed-off-by: Julio M. Merino Vidal <jmerino@ac.upc.edu>

diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 600e3d3..b8e8b8c 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -1,6 +1,21 @@
#ifndef __LINUX_PERCPU_H
#define __LINUX_PERCPU_H

+/*
+ * percpu provides a mechanism to define variables that are specific to
each
+ * CPU in the system.
+ *
+ * Each variable is defined as an independent array of NR_CPUS elements.
+ * This approach is used instead of a per-CPU structure because it has the
+ * following advantages:
+ * - Independent maintenance: a source file can define new per-CPU
+ * variables without distorting others.
+ * - 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.
+ */
+
#include <linux/spinlock.h> /* For preempt_disable() */
#include <linux/slab.h> /* For kmalloc() */
#include <linux/smp.h>

Kind regards.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-05-04 10:39    [from the cache]
©2003-2011 Jasper Spaans