Increase the generic ia64 config from its current max of 512 CPUs to a
new max of 1024 CPUs.
The principal file change that implements this is arch/ia64/defconfig,
which currently defines CONFIG_NR_CPUS=512. The change to
arch/ia64/Kconfig brings that file's declaration of the NR_CPUS default
up to the new reality of 1024. That Kconfig currently says
default "64"
which is bogus -- it is currently 512.
The recent change to arch/ia64/configs/sn2_defconfig just raised that
platform-specific defconfig to ==1024.
The rationale for increasing the generic ia64 defconfig to 1024 is that
the generic config needs to support the new upcoming max CPU count for
ia64 platforms, which for ia64/sn ("Altix") platform is 1024, just as
the current max of 512 supports the current max CPU count.
The downside is that the ia64 cpumask increases from 8 words to 16.
I have tried various heavy workloads and have seen no significant
measurable performance regression from this increase. The potential
extra cachemiss seems to be lost in the noise. The for_each_*cpu()
macros are relatively efficient in skipping past zeroed cpumask bits.
Workloads that impose higher loads on the CPU Scheduler tend to
bottleneck on non-Scheduler parts of the kernel, and it's the Scheduler
which makes the principal use of the cpumask_t, so these extra
cachemiss inefficiencies and extra CPU cycles to scan zero mask words
just get lost in the general system overhead.
Signed-off-by: John Hawkes <[email protected]>
Acked-by: Jack Steiner <[email protected]>
Acked-by: Greg Edwards <[email protected]>
Index: linux/arch/ia64/Kconfig
===================================================================
--- linux.orig/arch/ia64/Kconfig 2006-01-02 19:21:10.000000000 -0800
+++ linux/arch/ia64/Kconfig 2006-01-04 10:32:50.000000000 -0800
@@ -245,7 +245,7 @@
int "Maximum number of CPUs (2-1024)"
range 2 1024
depends on SMP
- default "64"
+ default "1024"
help
You should set this to the number of CPUs in your system, but
keep in mind that a kernel compiled for, e.g., 2 CPUs will boot but
Index: linux/arch/ia64/defconfig
===================================================================
--- linux.orig/arch/ia64/defconfig 2006-01-02 19:21:10.000000000 -0800
+++ linux/arch/ia64/defconfig 2006-01-04 10:32:50.000000000 -0800
@@ -98,7 +98,7 @@
# CONFIG_IA64_SGI_SN_XP is not set
CONFIG_FORCE_MAX_ZONEORDER=18
CONFIG_SMP=y
-CONFIG_NR_CPUS=512
+CONFIG_NR_CPUS=1024
CONFIG_HOTPLUG_CPU=y
# CONFIG_SCHED_SMT is not set
# CONFIG_PREEMPT is not set
-
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