From: "Robert M. Harris" <robert.m.harris@xxxxxxxxxx> __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(-) -- 1.8.3.1 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
