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SubjectRe: [PATCH] i386: Selectable Frequency of the Timer Interrupt.
On Mon, 16 May 2005, Andrew Morton wrote:

> So yes, the time has come around again to work out what we're going to do
> about this. I'd be a bit worried about allowing users to set HZ=724,
> simply because nobody tests with that, and it might expose odd timing
> relationships and unfortunate arithmetic rounding artifacts. So if we're
> going to do this thing it might be better to just offer 100, 250 and 1000.

Ok. Here is the patch allowing 100, 250 and 1000 HZ for i386 and x86_64:

-----
Make the timer frequency selectable. The timer interrupt may cause bus
and memory contention in large NUMA systems since the interrupt occurs
on each processor HZ times per second.

Signed-off-by: Christoph Lameter <christoph@lameter.com>
Signed-off-by: Shai Fultheim <shai@scalex86.org>

Index: linux-2.6.12-rc4/arch/i386/Kconfig
===================================================================
--- linux-2.6.12-rc4.orig/arch/i386/Kconfig 2005-05-17 02:19:55.000000000 +0000
+++ linux-2.6.12-rc4/arch/i386/Kconfig 2005-05-17 02:27:12.000000000 +0000
@@ -1133,6 +1133,20 @@
a work-around for a number of buggy BIOSes. Switch this option on if
your computer crashes instead of powering off properly.

+config HZ
+ int "Frequency of the Timer Interrupt (100, 250 or 1000 per second)"
+ range 100 1000
+ default 1000
+ help
+ Allows the configuration of the timer frequency. It is customary
+ to have the timer interrupt run at 1000 HZ but 100 HZ may be more
+ beneficial for servers and NUMA systems that do not need to have
+ a fast response for user interaction and that may experience bus
+ contention and cacheline bounces as a result of timer interrupts.
+ Note that the timer interrupt occurs on each processor in an SMP
+ environment leading to NR_CPUS * HZ number of timer interrupts
+ per second.
+
endmenu

source "arch/i386/kernel/cpu/cpufreq/Kconfig"
Index: linux-2.6.12-rc4/include/asm-i386/param.h
===================================================================
--- linux-2.6.12-rc4.orig/include/asm-i386/param.h 2005-05-17 02:15:57.000000000 +0000
+++ linux-2.6.12-rc4/include/asm-i386/param.h 2005-05-17 02:30:22.000000000 +0000
@@ -1,8 +1,19 @@
+#include <linux/config.h>
+
#ifndef _ASMi386_PARAM_H
#define _ASMi386_PARAM_H

#ifdef __KERNEL__
-# define HZ 1000 /* Internal kernel timer frequency */
+#if CONFIG_HZ == 1000
+#define HZ 1000 /* Internal kernel timer frequency */
+#elif CONFIG_HZ == 250
+#define CONFIG_HZ 250
+#elif CONFIG_HZ == 100
+#define HZ = 100
+#else
+#error "Invalid Timer Interrupt Frequency"
+#endif
+
# define USER_HZ 100 /* .. some user interfaces are in "ticks" */
# define CLOCKS_PER_SEC (USER_HZ) /* like times() */
#endif
Index: linux-2.6.12-rc4/arch/x86_64/Kconfig
===================================================================
--- linux-2.6.12-rc4.orig/arch/x86_64/Kconfig 2005-05-17 02:19:54.000000000 +0000
+++ linux-2.6.12-rc4/arch/x86_64/Kconfig 2005-05-17 02:26:38.000000000 +0000
@@ -410,6 +410,20 @@

If unsure, say Y. Only embedded should say N here.

+config HZ
+ int "Frequency of the Timer Interrupt (100, 250 or 1000 per second)"
+ range 100 1000
+ default 1000
+ help
+ Allows the configuration of the timer frequency. It is customary
+ to have the timer interrupt run at 1000 HZ but 100 HZ may be more
+ beneficial for servers and NUMA systems that do not need to have
+ a fast response for user interaction and that may experience bus
+ contention and cacheline bounces as a result of timer interrupts.
+ Note that the timer interrupt occurs on each processor in an SMP
+ environment leading to NR_CPUS * HZ number of timer interrupts
+ per second.
+
endmenu

#
Index: linux-2.6.12-rc4/include/asm-x86_64/param.h
===================================================================
--- linux-2.6.12-rc4.orig/include/asm-x86_64/param.h 2005-03-02 07:38:10.000000000 +0000
+++ linux-2.6.12-rc4/include/asm-x86_64/param.h 2005-05-17 02:28:04.000000000 +0000
@@ -1,8 +1,20 @@
+#include <linux/config.h>
+
#ifndef _ASMx86_64_PARAM_H
#define _ASMx86_64_PARAM_H

#ifdef __KERNEL__
-# define HZ 1000 /* Internal kernel timer frequency */
+
+#if CONFIG_HZ == 1000
+# define HZ 1000 /* Internal kernel timer frequency */
+#elif CONFIG_HZ == 250
+# define HZ 250
+#elif CONFIG_HZ == 100
+# define HZ 100
+#else
+#error "Invalid Timer Interrupt Frequency"
+#endif
+
# define USER_HZ 100 /* .. some user interfaces are in "ticks */
#define CLOCKS_PER_SEC (USER_HZ) /* like times() */
#endif
-
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