linux-kernel - [PATCH 01/10] mm: allow swappiness that prefers anon over file

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Date:	Mon,  6 Jun 2016 15:48:27 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michal Hocko <mhocko@...e.cz>,
	Tim Chen <tim.c.chen@...ux.intel.com>, kernel-team@...com
Subject: [PATCH 01/10] mm: allow swappiness that prefers anon over file

With the advent of fast random IO devices (SSDs, PMEM) and in-memory
swap devices such as zswap, it's possible for swap to be much faster
than filesystems, and for swapping to be preferable over thrashing
filesystem caches.

Allow setting swappiness - which defines the relative IO cost of cache
misses between page cache and swap-backed pages - to reflect such
situations by making the swap-preferred range configurable.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 Documentation/sysctl/vm.txt | 16 +++++++++++-----
 kernel/sysctl.c             |  3 ++-
 mm/vmscan.c                 |  2 +-
 3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 720355cbdf45..54030750cd31 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -771,14 +771,20 @@ with no ill effects: errors and warnings on these stats are suppressed.)
 
 swappiness
 
-This control is used to define how aggressive the kernel will swap
-memory pages.  Higher values will increase agressiveness, lower values
-decrease the amount of swap.  A value of 0 instructs the kernel not to
-initiate swap until the amount of free and file-backed pages is less
-than the high water mark in a zone.
+This control is used to define the relative IO cost of cache misses
+between the swap device and the filesystem as a value between 0 and
+200. At 100, the VM assumes equal IO cost and will thus apply memory
+pressure to the page cache and swap-backed pages equally. At 0, the
+kernel will not initiate swap until the amount of free and file-backed
+pages is less than the high watermark in a zone.
 
 The default value is 60.
 
+On non-rotational swap devices, a value of 100 (or higher, depending
+on what's backing the filesystem) is recommended.
+
+For in-memory swap, like zswap, values closer to 200 are recommended.
+
 ==============================================================
 
 - user_reserve_kbytes
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 2effd84d83e3..56a9243eb171 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -126,6 +126,7 @@ static int __maybe_unused two = 2;
 static int __maybe_unused four = 4;
 static unsigned long one_ul = 1;
 static int one_hundred = 100;
+static int two_hundred = 200;
 static int one_thousand = 1000;
 #ifdef CONFIG_PRINTK
 static int ten_thousand = 10000;
@@ -1323,7 +1324,7 @@ static struct ctl_table vm_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec_minmax,
 		.extra1		= &zero,
-		.extra2		= &one_hundred,
+		.extra2		= &two_hundred,
 	},
 #ifdef CONFIG_HUGETLB_PAGE
 	{
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c4a2f4512fca..f79010bbcdd4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -136,7 +136,7 @@ struct scan_control {
 #endif
 
 /*
- * From 0 .. 100.  Higher means more swappy.
+ * From 0 .. 200.  Higher means more swappy.
  */
 int vm_swappiness = 60;
 /*
-- 
2.8.3