Re: [PATCH 2/2] mm: page_alloc: High-order per-cpu page allocator v5

[Joonsoo Kim]

On Fri, Dec 02, 2016 at 09:21:08AM +0100, Michal Hocko wrote:
> On Fri 02-12-16 15:03:46, Joonsoo Kim wrote:
> [...]
> > > o pcp accounting during free is now confined to free_pcppages_bulk as it's
> > >   impossible for the caller to know exactly how many pages were freed.
> > >   Due to the high-order caches, the number of pages drained for a request
> > >   is no longer precise.
> > > 
> > > o The high watermark for per-cpu pages is increased to reduce the probability
> > >   that a single refill causes a drain on the next free.
> [...]
> > I guess that this patch would cause following problems.
> > 
> > 1. If pcp->batch is too small, high order page will not be freed
> > easily and survive longer. Think about following situation.
> > 
> > Batch count: 7
> > MIGRATE_UNMOVABLE -> MIGRATE_MOVABLE -> MIGRATE_RECLAIMABLE -> order 1
> > -> order 2...
> > 
> > free count: 1 + 1 + 1 + 2 + 4 = 9
> > so order 3 would not be freed.
> 
> I guess the second paragraph above in the changelog tries to clarify
> that...

It doesn't perfectly clarify my concern. This is a different problem.

>  
> > 2. And, It seems that this logic penalties high order pages. One free
> > to high order page means 1 << order pages free rather than just
> > one page free. This logic do round-robin to choose the target page so
> > amount of freed page will be different by the order.
> 
> Yes this is indeed possible. The first paragraph above mentions this
> problem.

Yes, it is mentioned simply but we cannot easily notice that the above
penalty for high order page is there.

Thanks.