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mm, numa: really disable NUMA balancing by default on single node machines

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NUMA balancing is meant to be disabled by default on UMA machines but
the check is using nr_node_ids (highest node) instead of
num_online_nodes (online nodes).

The consequences are that a UMA machine with a node ID of 1 or higher
will enable NUMA balancing.  This will incur useless overhead due to
minor faults with the impact depending on the workload.  These are the
impact on the stats when running a kernel build on a single node machine
whose node ID happened to be 1:

  			       vanilla     patched
  NUMA base PTE updates          5113158           0
  NUMA huge PMD updates              643           0
  NUMA page range updates        5442374           0
  NUMA hint faults               2109622           0
  NUMA hint local faults         2109622           0
  NUMA hint local percent            100         100
  NUMA pages migrated                  0           0

Signed-off-by: Mel Gorman <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Cc: <[email protected]>	[3.8+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Mel Gorman 2015-05-14 15:17:09 -07:00 • committed by Linus Torvalds
commit b0dc2b9bb4
1 changed files with 1 additions and 1 deletions
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2
mm/mempolicy.c
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@ -2518,7 +2518,7 @@ static void __init check_numabalancing_enable(void)
if (numabalancing_override)
set_numabalancing_state(numabalancing_override == 1);
if (nr_node_ids > 1 && !numabalancing_override) {
if (num_online_nodes() > 1 && !numabalancing_override) {
pr_info("%s automatic NUMA balancing. "
"Configure with numa_balancing= or the "
"kernel.numa_balancing sysctl",