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 rough relative IO cost of cache misses between page cache and swap-backed pages - to reflect such situations by making the swap-preferred range configurable. v2: clarify how to calculate swappiness (Minchan Kim) Signed-off-by: Johannes Weiner <[email protected]> --- Documentation/admin-guide/sysctl/vm.rst | 23 ++++++++++++++++++----- kernel/sysctl.c | 3 ++- mm/vmscan.c | 2 +- 3 files changed, 21 insertions(+), 7 deletions(-) diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index 0329a4d3fa9e..d46d5b7013c6 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -831,14 +831,27 @@ When page allocation performance is not a bottleneck and you want all swappiness ========== -This control is used to define how aggressive the kernel will swap -memory pages. Higher values will increase aggressiveness, 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 rough relative IO cost of swapping +and filesystem paging, 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; lower values signify more +expensive swap IO, higher values indicates cheaper. + +Keep in mind that filesystem IO patterns under memory pressure tend to +be more efficient than swap's random IO. An optimal value will require +experimentation and will also be workload-dependent. The default value is 60. +For in-memory swap, like zram or zswap, as well as hybrid setups that +have swap on faster devices than the filesystem, values beyond 100 can +be considered. For example, if the random IO against the swap device +is on average 2x faster than IO from the filesystem, swappiness should +be 133 (x + 2x = 200, 2x = 133.33). + +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. + unprivileged_userfaultfd ======================== diff --git a/kernel/sysctl.c b/kernel/sysctl.c index 8a176d8727a3..7f15d292e44c 100644 --- a/kernel/sysctl.c +++ b/kernel/sysctl.c @@ -131,6 +131,7 @@ static unsigned long zero_ul; static unsigned long one_ul = 1; static unsigned long long_max = LONG_MAX; static int one_hundred = 100; +static int two_hundred = 200; static int one_thousand = 1000; #ifdef CONFIG_PRINTK static int ten_thousand = 10000; @@ -1391,7 +1392,7 @@ static struct ctl_table vm_table[] = { .mode = 0644, .proc_handler = proc_dointvec_minmax, .extra1 = SYSCTL_ZERO, - .extra2 = &one_hundred, + .extra2 = &two_hundred, }, #ifdef CONFIG_HUGETLB_PAGE { diff --git a/mm/vmscan.c b/mm/vmscan.c index 70b0e2c6a4b9..43f88b1a4f14 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -161,7 +161,7 @@ struct scan_control { #endif /* - * From 0 .. 100. Higher means more swappy. + * From 0 .. 200. Higher means more swappy. */ int vm_swappiness = 60; /* -- 2.26.2
