linux-kernel - Re: [PATCH v6 08/11] mm, compaction: create compact

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Date:	Thu, 18 Aug 2016 14:13:49 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Hocko <mhocko@...nel.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 08/11] mm, compaction: create compact_gap wrapper

On 08/16/2016 08:41 AM, Joonsoo Kim wrote:
>> 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?

OK, Andrew can you add this -fix?
Thanks.

----8<----
>From cad7fb3a61c24feb53e8754248d67715b8ce0c3e Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@...e.cz>
Date: Thu, 18 Aug 2016 14:11:28 +0200
Subject: [PATCH] mm, compaction: create compact_gap wrapper-fix

Clarify the comment of compact_gap() wrt COMPACT_CLUSTER_MAX, per Joonsoo.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 include/linux/compaction.h | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index e7f0d34a90fe..585d55cb0dc0 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -67,9 +67,16 @@ 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.
+	 * free scanner 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. Note that the number of pages on the list is also
+	 * effectively limited by COMPACT_CLUSTER_MAX, as that's the maximum
+	 * that the migrate scanner can have isolated on migrate list, and free
+	 * scanner is only invoked when the number of isolated free pages is
+	 * lower than that. But it's not worth to complicate the formula here
+	 * as a bigger gap for higher orders than strictly necessary can also
+	 * improve chances of compaction success.
 	 */
 	return 2UL << order;
 }
-- 
2.9.2