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

[Michal Hocko]

On Sun 18-02-18 16:47:54, robert.m.harris@xxxxxxxxxx wrote:
> 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:

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