1.. _swap_numa:
2
3===========================================
4Automatically bind swap device to numa node
5===========================================
6
7If the system has more than one swap device and swap device has the node
8information, we can make use of this information to decide which swap
9device to use in get_swap_pages() to get better performance.
10
11
12How to use this feature
13=======================
14
15Swap device has priority and that decides the order of it to be used. To make
16use of automatically binding, there is no need to manipulate priority settings
17for swap devices. e.g. on a 2 node machine, assume 2 swap devices swapA and
18swapB, with swapA attached to node 0 and swapB attached to node 1, are going
19to be swapped on. Simply swapping them on by doing::
20
21	# swapon /dev/swapA
22	# swapon /dev/swapB
23
24Then node 0 will use the two swap devices in the order of swapA then swapB and
25node 1 will use the two swap devices in the order of swapB then swapA. Note
26that the order of them being swapped on doesn't matter.
27
28A more complex example on a 4 node machine. Assume 6 swap devices are going to
29be swapped on: swapA and swapB are attached to node 0, swapC is attached to
30node 1, swapD and swapE are attached to node 2 and swapF is attached to node3.
31The way to swap them on is the same as above::
32
33	# swapon /dev/swapA
34	# swapon /dev/swapB
35	# swapon /dev/swapC
36	# swapon /dev/swapD
37	# swapon /dev/swapE
38	# swapon /dev/swapF
39
40Then node 0 will use them in the order of::
41
42	swapA/swapB -> swapC -> swapD -> swapE -> swapF
43
44swapA and swapB will be used in a round robin mode before any other swap device.
45
46node 1 will use them in the order of::
47
48	swapC -> swapA -> swapB -> swapD -> swapE -> swapF
49
50node 2 will use them in the order of::
51
52	swapD/swapE -> swapA -> swapB -> swapC -> swapF
53
54Similaly, swapD and swapE will be used in a round robin mode before any
55other swap devices.
56
57node 3 will use them in the order of::
58
59	swapF -> swapA -> swapB -> swapC -> swapD -> swapE
60
61
62Implementation details
63======================
64
65The current code uses a priority based list, swap_avail_list, to decide
66which swap device to use and if multiple swap devices share the same
67priority, they are used round robin. This change here replaces the single
68global swap_avail_list with a per-numa-node list, i.e. for each numa node,
69it sees its own priority based list of available swap devices. Swap
70device's priority can be promoted on its matching node's swap_avail_list.
71
72The current swap device's priority is set as: user can set a >=0 value,
73or the system will pick one starting from -1 then downwards. The priority
74value in the swap_avail_list is the negated value of the swap device's
75due to plist being sorted from low to high. The new policy doesn't change
76the semantics for priority >=0 cases, the previous starting from -1 then
77downwards now becomes starting from -2 then downwards and -1 is reserved
78as the promoted value. So if multiple swap devices are attached to the same
79node, they will all be promoted to priority -1 on that node's plist and will
80be used round robin before any other swap devices.
81