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Swap devices are assumed to have similar accessing speed when swapon if no priority is specified. It's unfair and doesn't make sense just because one swap device is swapped on firstly, its priority will be higher than the one swapped on later. Here, set all swap devicess to have priority '-1' by default. With this change, swap device with default priority will be selected round robin when swapping out. This can improve the swapping efficiency a lot among multiple swap devices with default priority. Below are swapon output during the processes when high pressure vm-scability test is being taken: 1) This is pre-commita2468cc9bf, swap device is selectd one by one by priority from high to low when one swap device is exhausted: ------------------------------------ [root@hp-dl385g10-03 ~]# swapon NAME TYPE SIZE USED PRIO /dev/zram0 partition 16G 16G -1 /dev/zram1 partition 16G 966.2M -2 /dev/zram2 partition 16G 0B -3 /dev/zram3 partition 16G 0B -4 2) This is behaviour with commita2468cc9bf, on node, swap device sharing the same node id is selected firstly until exhausted; while on node no swap device sharing the node id it selects the one with highest priority until exhaustd: ------------------------------------ [root@hp-dl385g10-03 ~]# swapon NAME TYPE SIZE USED PRIO /dev/zram0 partition 16G 15.7G -2 /dev/zram1 partition 16G 3.4G -3 /dev/zram2 partition 16G 3.4G -4 /dev/zram3 partition 16G 2.6G -5 3) After this patch applied, swap devices with default priority are selectd round robin: ------------------------------------ [root@hp-dl385g10-03 block]# swapon NAME TYPE SIZE USED PRIO /dev/zram0 partition 16G 6.6G -1 /dev/zram1 partition 16G 6.6G -1 /dev/zram2 partition 16G 6.6G -1 /dev/zram3 partition 16G 6.6G -1 With the change, about 18% efficiency promotion relative to node based way as below. (Surely, the pre-commita2468cc9bfway is the worst.) vm-scability test: ================== Test with: usemem --init-time -O -y -x -n 31 2G (4G memcg, zram as swap) one by one: node based: round robin: System time: 1087.38 s 637.92 s 526.74 s (lower is better) Sum Throughput: 2036.55 MB/s 3546.56 MB/s 4207.56 MB/s (higher is better) Single process Throughput: 65.69 MB/s 114.40 MB/s 135.72 MB/s (high is better) free latency: 15769409.48 us 10138455.99 us 6810119.01 us(lower is better) Link: https://lkml.kernel.org/r/20251028034308.929550-3-bhe@redhat.com Signed-off-by: Baoquan He <bhe@redhat.com> Suggested-by: Chris Li <chrisl@kernel.org> Acked-by: Chris Li <chrisl@kernel.org> Acked-by: Nhat Pham <nphamcs@gmail.com> Cc: Barry Song <baohua@kernel.org> Cc: Kairui Song <kasong@tencent.com> Cc: Kemeng Shi <shikemeng@huaweicloud.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.