Re: [PATCH] mm, compaction: direct freepage allocation for async direct compaction

[Mel Gorman <mgorman@suse.de>]

On Wed, Nov 29, 2017 at 03:32:08PM +0900, Joonsoo Kim wrote:
> On Thu, Nov 23, 2017 at 02:08:43PM +0000, Mel Gorman wrote:
> 
> > 3. Another reason a linear scanner was used was because we wanted to
> >    clear entire pageblocks we were migrating from and pack the target
> >    pageblocks as much as possible. This was to reduce the amount of
> >    migration required overall even though the scanning hurts. This patch
> >    takes MIGRATE_MOVABLE pages from anywhere that is "not this pageblock".
> >    Those potentially have to be moved again and again trying to randomly
> >    fill a MIGRATE_MOVABLE block. Have you considered using the freelists
> >    as a hint? i.e. take a page from the freelist, then isolate all free
> >    pages in the same pageblock as migration targets? That would preserve
> >    the "packing property" of the linear scanner.
> > 
> >    This would increase the amount of scanning but that *might* be offset by
> >    the number of migrations the workload does overall. Note that migrations
> >    potentially are minor faults so if we do too many migrations, your
> >    workload may suffer.
> > 
> > 4. One problem the linear scanner avoids is that a migration target is
> >    subsequently used as a migration source and leads to a ping-pong effect.
> >    I don't know how bad this is in practice or even if it's a problem at
> >    all but it was a concern at the time
> 
> IIUC, this potential advantage for a linear scanner would not be the
> actual advantage in the *running* system.
> 
> Consider about following worst case scenario for "direct freepage
> allocation" that "moved again" happens.
> 

The immediate case should be ok as long as the migration source and the
pageblock a freepage is taken from is not the same pageblock. That might
mean that more pages from the freelist would need to be examined until
another pageblock was found.

> 
> So, I think that "direct freepage allocation" doesn't suffer from such
> a ping-poing effect. Am I missing something?
> 

The ping-pong effect I'm concerned with is that a previous migration
target is used as a migration source in the future. It's hard for that
situation to occur with two linear scanners but care is needed when
using direct freepage allocation.

-- 
Mel Gorman
SUSE Labs

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