Re: [RFC 13/13] mm, compaction: fix and improve watermark handling

[Mel Gorman]

On Wed, May 18, 2016 at 04:27:53PM +0200, Michal Hocko wrote:
> > > > - __compaction_suitable() then checks the low watermark plus a (2 << order) gap
> > > >   to decide if there's enough free memory to perform compaction. This check
> > > 
> > > And this was a real head scratcher when I started looking into the
> > > compaction recently. Why do we need to be above low watermark to even
> > > start compaction. Compaction uses additional memory only for a short
> > > period of time and then releases the already migrated pages.
> > > 
> > 
> > Simply minimising the risk that compaction would deplete the entire
> > zone. Sure, it hands pages back shortly afterwards. At the time of the
> > initial prototype, page migration was severely broken and the system was
> > constantly crashing. The cautious checks were left in place after page
> > migration was fixed as there wasn't a compelling reason to remove them
> > at the time.
> 
> OK, then moving to min_wmark + bias from low_wmark should work, right?

Yes. I did recall there was another reason but it's marginal. I didn't
want compaction isolation free pages to artifically push a process into
direct reclaim but given that we are likely under memory pressure at
that time anyway, it's unlikely that compaction is the sole reason
processes are entering direct reclaim.

-- 
Mel Gorman
SUSE Labs