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

[Robert Harris]

> On 19 Feb 2018, at 13:10, Mel Gorman <mgorman@xxxxxxx> wrote:
> 
> 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.

Understood.  Thanks.

Robert Harris