.. _swap_numa:
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===========================================
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Automatically bind swap device to numa node
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===========================================
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If the system has more than one swap device and swap device has the node
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information, we can make use of this information to decide which swap
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device to use in get_swap_pages() to get better performance.
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How to use this feature
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=======================
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Swap device has priority and that decides the order of it to be used. To make
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use of automatically binding, there is no need to manipulate priority settings
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for swap devices. e.g. on a 2 node machine, assume 2 swap devices swapA and
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swapB, with swapA attached to node 0 and swapB attached to node 1, are going
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to be swapped on. Simply swapping them on by doing::
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# swapon /dev/swapA
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# swapon /dev/swapB
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Then node 0 will use the two swap devices in the order of swapA then swapB and
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node 1 will use the two swap devices in the order of swapB then swapA. Note
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that the order of them being swapped on doesn't matter.
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A more complex example on a 4 node machine. Assume 6 swap devices are going to
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be swapped on: swapA and swapB are attached to node 0, swapC is attached to
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node 1, swapD and swapE are attached to node 2 and swapF is attached to node3.
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The way to swap them on is the same as above::
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# swapon /dev/swapA
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# swapon /dev/swapB
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# swapon /dev/swapC
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# swapon /dev/swapD
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# swapon /dev/swapE
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# swapon /dev/swapF
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Then node 0 will use them in the order of::
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swapA/swapB -> swapC -> swapD -> swapE -> swapF
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swapA and swapB will be used in a round robin mode before any other swap device.
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node 1 will use them in the order of::
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swapC -> swapA -> swapB -> swapD -> swapE -> swapF
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node 2 will use them in the order of::
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swapD/swapE -> swapA -> swapB -> swapC -> swapF
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Similaly, swapD and swapE will be used in a round robin mode before any
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other swap devices.
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node 3 will use them in the order of::
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swapF -> swapA -> swapB -> swapC -> swapD -> swapE
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Implementation details
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======================
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The current code uses a priority based list, swap_avail_list, to decide
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which swap device to use and if multiple swap devices share the same
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priority, they are used round robin. This change here replaces the single
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global swap_avail_list with a per-numa-node list, i.e. for each numa node,
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it sees its own priority based list of available swap devices. Swap
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device's priority can be promoted on its matching node's swap_avail_list.
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The current swap device's priority is set as: user can set a >=0 value,
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or the system will pick one starting from -1 then downwards. The priority
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value in the swap_avail_list is the negated value of the swap device's
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due to plist being sorted from low to high. The new policy doesn't change
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the semantics for priority >=0 cases, the previous starting from -1 then
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downwards now becomes starting from -2 then downwards and -1 is reserved
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as the promoted value. So if multiple swap devices are attached to the same
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node, they will all be promoted to priority -1 on that node's plist and will
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be used round robin before any other swap devices.
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