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 <han...@cmpxchg.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
