[PATCH 0/1] mm, compaction: correct the bounds of __fragmentation_index()

robert.m.harris
2018-02-18T16:47:54Z

From: "Robert M. Harris" <robert.m.harris@oracle.com>

__fragmentation_index() calculates a value used to determine whether
compaction should be favoured over page reclaim in the event of
allocation failure. The function purports to return a value between 0
and 1000, representing units of 1/1000. Barring the case of a
pathological shortfall of memory, the lower bound is instead 500. This
is significant because it is the default value of
sysctl_extfrag_threshold, i.e. the value below which compaction should
be avoided in favour of page reclaim for costly pages.

Here's an illustration using a zone that I fragmented with selective
calls to __alloc_pages() and __free_pages --- the fragmentation for
order-1 could not be minimised further yet is reported as 0.5:

# head -1 /proc/buddyinfo
Node 0, zone DMA 1983 0 0 0 0 0 0 0 0 0 0
# head -1 /sys/kernel/debug/extfrag/extfrag_index
Node 0, zone DMA -1.000 0.500 0.750 0.875 0.937 0.969 0.984 0.992 0.996 0.998 0.999
#

With extreme memory shortage the reported fragmentation index does go
lower. In fact, it can go below zero:

# head -1 /proc/buddyinfo
Node 0, zone DMA 1 0 0 0 0 0 0 0 0 0 0
# head -1 /sys/kernel/debug/extfrag/extfrag_index
Node 0, zone DMA -1.000 0.-500 0.-250 0.-125 0.-62 0.-31 0.-15 0.-07 0.-03 0.-01 0.000
#

This patch implements and documents a modified version of the original
expression that returns a value in the range 0 <= index < 1000. It
amends the default value of sysctl_extfrag_threshold to preserve the
existing behaviour. With this patch in place, the same two tests yield

# head -1 /proc/buddyinfo
Node 0, zone DMA 1983 0 0 0 0 0 0 0 0 0 0
# head -1 /sys/kernel/debug/extfrag/extfrag_index
Node 0, zone DMA -1.000 0.000 0.500 0.750 0.875 0.937 0.969 0.984 0.992 0.996 0.998
#

and

# head -1 /proc/buddyinfo
Node 0, zone DMA 1 0 0 0 0 0 0 0 0 0 0
# head -1 /sys/kernel/debug/extfrag/extfrag_index
Node 0, zone DMA -1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
#

Robert M. Harris (1):
mm, compaction: correct the bounds of __fragmentation_index()

Documentation/sysctl/vm.txt | 2 +-
mm/compaction.c | 2 +-
mm/vmstat.c | 47 +++++++++++++++++++++++++++++++++++----------
3 files changed, 39 insertions(+), 12 deletions(-)

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1.8.3.1