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SubjectRe: [patch 1/2] mm, compaction: kcompactd should not ignore pageblock skip
On Wed, 23 Aug 2017, Vlastimil Babka wrote:

> On 08/16/2017 01:39 AM, David Rientjes wrote:
> > Kcompactd is needlessly ignoring pageblock skip information. It is doing
> > MIGRATE_SYNC_LIGHT compaction, which is no more powerful than
> > MIGRATE_SYNC compaction.
> >
> > If compaction recently failed to isolate memory from a set of pageblocks,
> > there is nothing to indicate that kcompactd will be able to do so, or
> > that it is beneficial from attempting to isolate memory.
> >
> > Use the pageblock skip hint to avoid rescanning pageblocks needlessly
> > until that information is reset.
> >
> > Signed-off-by: David Rientjes <rientjes@google.com>
>
> It would be much better if patches like this were accompanied by some
> numbers.
>

The numbers were from https://marc.info/?l=linux-mm&m=150231232707999
where very large amounts (>90% of system RAM) were hugetlb pages. We can
supplement this changelog with the following if it helps:

"""
Currently, kcompactd ignores pageblock skip information that can become
useful if it is known that memory should not be considered by both the
migration and freeing scanners. Abundant hugetlb memory is a good example
of memory that is needlessly (and expensively) scanned since the hugepage
order normally matches the pageblock order.

For example, on a sysctl with very large amounts of memory reserved by the
hugetlb subsystem:

compact_migrate_scanned 2931254031294
compact_free_scanned 102707804816705
compact_isolated 1309145254

Kcompactd ends up successfully isolating ~0.0012% of memory that is
scans (the above does not involve direct compaction).

A follow-up change will set the pageblock skip for this memory since it is
never useful for either scanner.
"""

> Also there's now a danger that in cases where there's no direct
> compaction happening (just kcompactd), nothing will ever call
> __reset_isolation_suitable().
>

I'm not sure that is helpful in a context where no high-order memory can
call direct compaction that kcompactd needlessly scanning the same memory
over and over is beneficial.

> > ---
> > mm/compaction.c | 3 +--
> > 1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -1927,9 +1927,8 @@ static void kcompactd_do_work(pg_data_t *pgdat)
> > .total_free_scanned = 0,
> > .classzone_idx = pgdat->kcompactd_classzone_idx,
> > .mode = MIGRATE_SYNC_LIGHT,
> > - .ignore_skip_hint = true,
> > + .ignore_skip_hint = false,
> > .gfp_mask = GFP_KERNEL,
> > -
> > };
> > trace_mm_compaction_kcompactd_wake(pgdat->node_id, cc.order,
> > cc.classzone_idx);
> >
>
>

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