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

Mel Gorman
2018-02-19T13:10:24Z

On Mon, Feb 19, 2018 at 12:26:39PM +0000, Robert Harris wrote:
>
>
> > On 19 Feb 2018, at 09:47, Mel Gorman <mgorman@suse.de> wrote:
> >
> > On Sun, Feb 18, 2018 at 04:47:55PM +0000, robert.m.harris@oracle.com wrote:
> >> 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 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@oracle.com>
> >
> > 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