mm: disable zone_reclaim_mode by default

When it was introduced, zone_reclaim_mode made sense as NUMA distances
punished and workloads were generally partitioned to fit into a NUMA
node.  NUMA machines are now common but few of the workloads are
NUMA-aware and it's routine to see major performance degradation due to
zone_reclaim_mode being enabled but relatively few can identify the
problem.

Those that require zone_reclaim_mode are likely to be able to detect
when it needs to be enabled and tune appropriately so lets have a
sensible default for the bulk of users.

This patch (of 2):

zone_reclaim_mode causes processes to prefer reclaiming memory from
local node instead of spilling over to other nodes.  This made sense
initially when NUMA machines were almost exclusively HPC and the
workload was partitioned into nodes.  The NUMA penalties were
sufficiently high to justify reclaiming the memory.  On current machines
and workloads it is often the case that zone_reclaim_mode destroys
performance but not all users know how to detect this.  Favour the
common case and disable it by default.  Users that are sophisticated
enough to know they need zone_reclaim_mode will detect it.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index dd9d0e33b44391200c2004584e79d66baf527991..5b6da0fb5fbfbf7a585fe40973e66c10ec519fe2 100644 (file)
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -772,16 +772,17 @@ This is value ORed together of
 2      = Zone reclaim writes dirty pages out
 4      = Zone reclaim swaps pages
 
-zone_reclaim_mode is set during bootup to 1 if it is determined that pages
-from remote zones will cause a measurable performance reduction. The
-page allocator will then reclaim easily reusable pages (those page
-cache pages that are currently not used) before allocating off node pages.
-
-It may be beneficial to switch off zone reclaim if the system is
-used for a file server and all of memory should be used for caching files
-from disk. In that case the caching effect is more important than
+zone_reclaim_mode is disabled by default.  For file servers or workloads
+that benefit from having their data cached, zone_reclaim_mode should be
+left disabled as the caching effect is likely to be more important than
 data locality.
 
+zone_reclaim may be enabled if it's known that the workload is partitioned
+such that each partition fits within a NUMA node and that accessing remote
+memory would cause a measurable performance reduction.  The page allocator
+will then reclaim easily reusable pages (those page cache pages that are
+currently not used) before allocating off node pages.
+
 Allowing zone reclaim to write out pages stops processes that are
 writing large amounts of data from dirtying pages on other nodes. Zone
 reclaim will write out dirty pages if a zone fills up and so effectively

diff --git a/arch/ia64/include/asm/topology.h b/arch/ia64/include/asm/topology.h
index 3202aa74e0d62838e104b0497e8a8bb6080aa92f..6437ca21f61b49d0b05fe9d124ed0e5aa8391d92 100644 (file)
--- a/arch/ia64/include/asm/topology.h
+++ b/arch/ia64/include/asm/topology.h
@@ -21,7 +21,8 @@
 #define PENALTY_FOR_NODE_WITH_CPUS 255
 
 /*
- * Distance above which we begin to use zone reclaim
+ * Nodes within this distance are eligible for reclaim by zone_reclaim() when
+ * zone_reclaim_mode is enabled.
  */
 #define RECLAIM_DISTANCE 15
 

diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index c9202151079f2b1d6e99af4569b82ce68c785573..6c8a8c5a37a1cc761386ba89db9e6136fca5f3c2 100644 (file)
--- a/arch/powerpc/include/asm/topology.h
+++ b/arch/powerpc/include/asm/topology.h
@@ -9,12 +9,8 @@ struct device_node;
 #ifdef CONFIG_NUMA
 
 /*
- * Before going off node we want the VM to try and reclaim from the local
- * node. It does this if the remote distance is larger than RECLAIM_DISTANCE.
- * With the default REMOTE_DISTANCE of 20 and the default RECLAIM_DISTANCE of
- * 20, we never reclaim and go off node straight away.
- *
- * To fix this we choose a smaller value of RECLAIM_DISTANCE.
+ * If zone_reclaim_mode is enabled, a RECLAIM_DISTANCE of 10 will mean that
+ * all zones on all nodes will be eligible for zone_reclaim().
  */
 #define RECLAIM_DISTANCE 10
 

diff --git a/include/linux/topology.h b/include/linux/topology.h
index 973671ff9e7d9d433b8924762b8c3090d1cebc18..dda6ee521e74e3fefff78e17b54624fa53ffae96 100644 (file)
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -58,7 +58,8 @@ int arch_update_cpu_topology(void);
 /*
  * If the distance between nodes in a system is larger than RECLAIM_DISTANCE
  * (in whatever arch specific measurement units returned by node_distance())
- * then switch on zone reclaim on boot.
+ * and zone_reclaim_mode is enabled then the VM will only call zone_reclaim()
+ * on nodes within this distance.
  */
 #define RECLAIM_DISTANCE 30
 #endif

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7cfdcd808f520ab91f527f750ced42ec338d6a9b..dfe954fbb48aeda3c6a2ec91bd1c18c158ec29da 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1860,8 +1860,6 @@ static void __paginginit init_zone_allows_reclaim(int nid)
        for_each_node_state(i, N_MEMORY)
                if (node_distance(nid, i) <= RECLAIM_DISTANCE)
                        node_set(i, NODE_DATA(nid)->reclaim_nodes);
-               else
-                       zone_reclaim_mode = 1;
 }
 
 #else  /* CONFIG_NUMA */