Re: [PATCH v6 08/11] mm, compaction: create compact_gap wrapper

[Joonsoo Kim]

On Tue, Aug 16, 2016 at 08:15:36AM +0200, Vlastimil Babka wrote:
> On 08/16/2016 08:15 AM, Joonsoo Kim wrote:
> >On Wed, Aug 10, 2016 at 11:12:23AM +0200, Vlastimil Babka wrote:
> >>--- a/include/linux/compaction.h
> >>+++ b/include/linux/compaction.h
> >>@@ -58,6 +58,22 @@ enum compact_result {
> >>
> >> struct alloc_context; /* in mm/internal.h */
> >>
> >>+/*
> >>+ * Number of free order-0 pages that should be available above given watermark
> >>+ * to make sure compaction has reasonable chance of not running out of free
> >>+ * pages that it needs to isolate as migration target during its work.
> >>+ */
> >>+static inline unsigned long compact_gap(unsigned int order)
> >>+{
> >>+	/*
> >>+	 * Although all the isolations for migration are temporary, compaction
> >>+	 * may have up to 1 << order pages on its list and then try to split
> >>+	 * an (order - 1) free page. At that point, a gap of 1 << order might
> >>+	 * not be enough, so it's safer to require twice that amount.
> >>+	 */
> >>+	return 2UL << order;
> >>+}
> >
> >I agree with this wrapper function but there is a question.
> >
> >Could you elaborate more on this code comment? Freescanner could keep
> >COMPACT_CLUSTER_MAX freepages on the list. It's not associated with
> >requested order at least for now. Why compact_gap is 2UL << order in
> >this case?
> 
> It's true that for high enough order, COMPACT_CLUSTER_MAX might be
> more limiting than 1 << order. But then it also helps to have more

AFAIK, regardless of order, migration scanner isolates
COMPACT_CLUSTER_MAX pages. And, freepage scanner isolates
more than nr_migratepages freepages.

> free pages for probability of compaction success, so I don't think
> it's worth complicating the compact_gap() formula.

I agree that it's not worth complicating the compact_gap() formula but
it would be better to fix the comment?

Thanks.