On Sun 18-02-18 16:47:54, robert.m.har...@oracle.com wrote: > From: "Robert M. Harris" <robert.m.har...@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:
Cover letter for a single patch is usually an overkill. Why is this information not valuable in the patch description directly? > # 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 > -- Michal Hocko SUSE Labs
