How to use this feature¶

Swap device has priority and that decides the order of it to be used. To makeuse of automatically binding, there is no need to manipulate priority settingsfor swap devices. e.g. on a 2 node machine, assume 2 swap devices swapA andswapB, with swapA attached to node 0 and swapB attached to node 1, are goingto be swapped on. Simply swapping them on by doing:

# swapon /dev/swapA# swapon /dev/swapB

Then node 0 will use the two swap devices in the order of swapA then swapB andnode 1 will use the two swap devices in the order of swapB then swapA. Notethat the order of them being swapped on doesn’t matter.

A more complex example on a 4 node machine. Assume 6 swap devices are going tobe swapped on: swapA and swapB are attached to node 0, swapC is attached tonode 1, swapD and swapE are attached to node 2 and swapF is attached to node3.The way to swap them on is the same as above:

# swapon /dev/swapA# swapon /dev/swapB# swapon /dev/swapC# swapon /dev/swapD# swapon /dev/swapE# swapon /dev/swapF

Then node 0 will use them in the order of:

swapA/swapB -> swapC -> swapD -> swapE -> swapF

swapA and swapB will be used in a round robin mode before any other swap device.

node 1 will use them in the order of:

swapC -> swapA -> swapB -> swapD -> swapE -> swapF

node 2 will use them in the order of:

swapD/swapE -> swapA -> swapB -> swapC -> swapF

Similaly, swapD and swapE will be used in a round robin mode before anyother swap devices.

node 3 will use them in the order of:

swapF -> swapA -> swapB -> swapC -> swapD -> swapE

Implementation details¶

The current code uses a priority based list, swap_avail_list, to decidewhich swap device to use and if multiple swap devices share the samepriority, they are used round robin. This change here replaces the singleglobal swap_avail_list with a per-numa-node list, i.e. for each numa node,it sees its own priority based list of available swap devices. Swapdevice’s priority can be promoted on its matching node’s swap_avail_list.

The current swap device’s priority is set as: user can set a >=0 value,or the system will pick one starting from -1 then downwards. The priorityvalue in the swap_avail_list is the negated value of the swap device’sdue to plist being sorted from low to high. The new policy doesn’t changethe semantics for priority >=0 cases, the previous starting from -1 thendownwards now becomes starting from -2 then downwards and -1 is reservedas the promoted value. So if multiple swap devices are attached to the samenode, they will all be promoted to priority -1 on that node’s plist and willbe used round robin before any other swap devices.