Re: [PATCH 4/4] zone_reclaim_mode is always 0 by default

Robin Holt
2009-05-14T11:48:38Z

> Unfortunately no.
> zone reclaim has two weakness by design.
>
> 1.
> zone reclaim don't works well when workingset size > local node size.
> but it can happen easily on small machine.
> if it happen, zone reclaim drop own process's memory.
>
> Plus, zone reclaim also doesn't fit DB server. its process has large
> workingset.

Large DB server is not your typical desktop application either.

> 2.
> zone reclaim have inter zone balancing issue.
>
> example: x86_64 2node 8G machine has following zone assignment
>
> zone 0 (DMA32): 3GB
> zone 0 (Normal): 1GB
> zone 1 (Normal): 4GB
>
> if the page is allocated from DMA32, you are lucky. DMA32 isn't reclaimed
> so freqently. but if from zone0 Normal, you are unlucky.
> it is very frequent reclaimed although it is small than other zone.

I have seen that behavior on some of our mismatched large systems as well,
although never had one so imbalanced because ia64 only has Normal.

> I know my patch change large server default. but I believe linux
> default kernel parameter adapt to desktop and entry machine.

If this imbalance is an x86_64 only problem, then we could do something
simple like the following untested patch. This leaves the default
for everyone except x86_64.

Robin

------------------------------------------------------------------------

Even if there is a great node distance on x86_64, disable zone reclaim
by default. This was done to handle the imbalanced zone sizes where a
majority of the memory in zone 0 is DMA32 with a small remaining Normal
which will be aggressively reclaimed.

For other architectures, we leave the default behavior.

Signed-off-by: Robin Holt <holt@sgi.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Rik van Riel <riel@redhat.com>

---
arch/x86/include/asm/topology.h | 2 ++
include/linux/topology.h | 5 +++++
mm/page_alloc.c | 2 +-
3 files changed, 8 insertions(+), 1 deletion(-)
Index: page_reclaim_mode/arch/x86/include/asm/topology.h
===================================================================
--- page_reclaim_mode.orig/arch/x86/include/asm/topology.h 2009-05-14 06:44:20.118925713 -0500
+++ page_reclaim_mode/arch/x86/include/asm/topology.h 2009-05-14 06:44:21.251067716 -0500
@@ -128,6 +128,8 @@ extern unsigned long node_remap_size[];

#endif

+#define DEFAULT_ZONE_RECLAIM_MODE 0
+
/* sched_domains SD_NODE_INIT for NUMA machines */
#define SD_NODE_INIT (struct sched_domain) { \
.min_interval = 8, \
Index: page_reclaim_mode/include/linux/topology.h
===================================================================
--- page_reclaim_mode.orig/include/linux/topology.h 2009-05-14 06:44:20.070919619 -0500
+++ page_reclaim_mode/include/linux/topology.h 2009-05-14 06:44:21.279071382 -0500
@@ -61,6 +61,11 @@ int arch_update_cpu_topology(void);
*/
#define RECLAIM_DISTANCE 20
#endif
+
+#ifndef DEFAULT_ZONE_RECLAIM_MODE
+#define DEFAULT_ZONE_RECLAIM_MODE 1
+#endif
+
#ifndef PENALTY_FOR_NODE_WITH_CPUS
#define PENALTY_FOR_NODE_WITH_CPUS (1)
#endif
Index: page_reclaim_mode/mm/page_alloc.c
===================================================================
--- page_reclaim_mode.orig/mm/page_alloc.c 2009-05-14 06:44:20.138928363 -0500
+++ page_reclaim_mode/mm/page_alloc.c 2009-05-14 06:44:21.311075244 -0500
@@ -2331,7 +2331,7 @@ static void build_zonelists(pg_data_t *p
* to reclaim pages in a zone before going off node.
*/
if (distance > RECLAIM_DISTANCE)
- zone_reclaim_mode = 1;
+ zone_reclaim_mode = DEFAULT_ZONE_RECLAIM_MODE;

/*
* We don't want to pressure a particular node.