Re: kswapd skips compaction if reclaim order drops to zero?

[Mel Gorman <mgorman@suse.de>]

On Thu, Aug 15, 2013 at 10:41:39PM +0900, Minchan Kim wrote:
> Hey Mel,
> 
> On Thu, Aug 15, 2013 at 11:47:27AM +0100, Mel Gorman wrote:
> > On Thu, Aug 15, 2013 at 06:02:53PM +0800, Hillf Danton wrote:
> > > If the allocation order is not high, direct compaction does nothing.
> > > Can we skip compaction here if order drops to zero?
> > > 
> > 
> > If the allocation order is not high then
> > 
> > pgdat_needs_compaction == (order > 0) == false == no calling compact_pdatt
> > 
> > In the case where order is reset to 0 due to fragmentation then it does
> > call compact_pgdat but it does no work due to the cc->order check in
> > __compact_pgdat.
> > 
> 
> I am looking at mmotm-2013-08-07-16-55 but couldn't find cc->order
> check right before compact_zone in __comact_pgdat.
> Could you pinpoint code piece?
> 

Thanks, I screwed up as that check happens too late. However, it still
ends up not mattering because it does this

compact_pgdat
  -> __compact_pgdat
    -> compact_zone
      -> compaction_suitable

For order == 0, compaction_suitable will return either COMPACT_SKIPPED
(if the watermarks are not met) and COMPACT_PARTIAL otherwise. Either
way, compaction doesn't run.

-- 
Mel Gorman
SUSE Labs