Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 Automatically bind swap device to numa node 2 ------------------------------------------- 3 4 If the system has more than one swap device and swap device has the node 5 information, we can make use of this information to decide which swap 6 device to use in get_swap_pages() to get better performance. 7 8 9 How to use this feature 10 ----------------------- 11 12 Swap device has priority and that decides the order of it to be used. To make 13 use of automatically binding, there is no need to manipulate priority settings 14 for swap devices. e.g. on a 2 node machine, assume 2 swap devices swapA and 15 swapB, with swapA attached to node 0 and swapB attached to node 1, are going 16 to be swapped on. Simply swapping them on by doing: 17 # swapon /dev/swapA 18 # swapon /dev/swapB 19 20 Then node 0 will use the two swap devices in the order of swapA then swapB and 21 node 1 will use the two swap devices in the order of swapB then swapA. Note 22 that the order of them being swapped on doesn't matter. 23 24 A more complex example on a 4 node machine. Assume 6 swap devices are going to 25 be swapped on: swapA and swapB are attached to node 0, swapC is attached to 26 node 1, swapD and swapE are attached to node 2 and swapF is attached to node3. 27 The way to swap them on is the same as above: 28 # swapon /dev/swapA 29 # swapon /dev/swapB 30 # swapon /dev/swapC 31 # swapon /dev/swapD 32 # swapon /dev/swapE 33 # swapon /dev/swapF 34 35 Then node 0 will use them in the order of: 36 swapA/swapB -> swapC -> swapD -> swapE -> swapF 37 swapA and swapB will be used in a round robin mode before any other swap device. 38 39 node 1 will use them in the order of: 40 swapC -> swapA -> swapB -> swapD -> swapE -> swapF 41 42 node 2 will use them in the order of: 43 swapD/swapE -> swapA -> swapB -> swapC -> swapF 44 Similaly, swapD and swapE will be used in a round robin mode before any 45 other swap devices. 46 47 node 3 will use them in the order of: 48 swapF -> swapA -> swapB -> swapC -> swapD -> swapE 49 50 51 Implementation details 52 ---------------------- 53 54 The current code uses a priority based list, swap_avail_list, to decide 55 which swap device to use and if multiple swap devices share the same 56 priority, they are used round robin. This change here replaces the single 57 global swap_avail_list with a per-numa-node list, i.e. for each numa node, 58 it sees its own priority based list of available swap devices. Swap 59 device's priority can be promoted on its matching node's swap_avail_list. 60 61 The current swap device's priority is set as: user can set a >=0 value, 62 or the system will pick one starting from -1 then downwards. The priority 63 value in the swap_avail_list is the negated value of the swap device's 64 due to plist being sorted from low to high. The new policy doesn't change 65 the semantics for priority >=0 cases, the previous starting from -1 then 66 downwards now becomes starting from -2 then downwards and -1 is reserved 67 as the promoted value. So if multiple swap devices are attached to the same 68 node, they will all be promoted to priority -1 on that node's plist and will 69 be used round robin before any other swap devices.