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

[Mel Gorman]

On Mon, Feb 19, 2018 at 12:26:39PM +0000, Robert Harris wrote:
> 
> 
> > On 19 Feb 2018, at 09:47, Mel Gorman <mgorman@xxxxxxx> wrote:
> > 
> > On Sun, Feb 18, 2018 at 04:47:55PM +0000, 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 calculation itself is opaque and, on inspection, does not
> >> match its existing description.  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.
> >> 
> >> 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.
> >> 
> >> Signed-off-by: Robert M. Harris <robert.m.harris@xxxxxxxxxx>
> > 
> > You have to update sysctl_extfrag_threshold as well for the new bounds.
> 
> This patch makes its default value zero.
> 

Sorry, I'm clearly blind.

> > It effectively makes it a no-op but it was a no-op already and adjusting
> > that default should be supported by data indicating it's safe.
> 
> Would it be acceptable to demonstrate using tracing that in both the
> pre- and post-patch cases
> 
>   1. compaction is attempted regardless of fragmentation index,
>      excepting that
> 
>   2. reclaim is preferred even for non-zero fragmentation during
>      an extreme shortage of memory
> 

If you can demonstrate that for both reclaim-intensive and
compaction-intensive workloads then yes. Also include the reclaim and
compaction stats from /proc/vmstat and not just tracepoints to demonstrate
that reclaim doesn't get out of control and reclaim the world in
response to failed high-order allocations such as THP.

-- 
Mel Gorman
SUSE Labs