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Subject[PATCH 25/52] sched, numa: Improve the CONFIG_NUMA_BALANCING help text
Date
Add information about cost, type of optimizations, etc.,
so that the user knows what to expect.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
init/Kconfig | 33 +++++++++++++++++++++++++++++++--
1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index 37cd8d6..018c8af 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -721,13 +721,42 @@ config ARCH_USES_NUMA_PROT_NONE
depends on NUMA_BALANCING

config NUMA_BALANCING
- bool "Memory placement aware NUMA scheduler"
+ bool "NUMA-optimizing scheduler"
default n
depends on ARCH_SUPPORTS_NUMA_BALANCING
depends on !ARCH_WANT_NUMA_VARIABLE_LOCALITY
depends on SMP && NUMA && MIGRATION
help
- This option adds support for automatic NUMA aware memory/task placement.
+ This option enables NUMA-aware, transparent, automatic
+ placement optimizations of memory, tasks and task groups.
+
+ The optimizations work by (transparently) runtime sampling the
+ workload sharing relationship between threads and processes
+ of long-run workloads, and scheduling them based on these
+ measured inter-task relationships (or the lack thereof).
+
+ ("Long-run" means several seconds of CPU runtime at least.)
+
+ Tasks that predominantly perform their own processing, without
+ interacting with other tasks much will be independently balanced
+ to a CPU and their working set memory will migrate to that CPU/node.
+
+ Tasks that share a lot of data with each other will be attempted to
+ be scheduled on as few nodes as possible, with their memory
+ following them there and being distributed between those nodes.
+
+ This optimization can improve the performance of long-run CPU-bound
+ workloads by 10% or more. The sampling and migration has a small
+ but nonzero cost, so if your NUMA workload is already perfectly
+ placed (for example by use of explicit CPU and memory bindings,
+ or because the stock scheduler does a good job already) then you
+ probably don't need this feature.
+
+ [ On non-NUMA systems this feature will not be active. You can query
+ whether your system is a NUMA system via looking at the output of
+ "numactl --hardware". ]
+
+ Say N if unsure.

menuconfig CGROUPS
boolean "Control Group support"
--
1.7.11.7


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