Re: [PATCH v5 mm-new 2/2] mm/swap: select swap device with default priority round robin

[Nhat Pham <nphamcs@gmail.com>]

On Mon, Oct 27, 2025 at 8:43 PM Baoquan He <bhe@redhat.com> wrote:
>
> 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.

Yeah, I agree. In a set up with multiple swap devices, this can spread
out the swap writes, potentially improving throughput.

>
> Below are swapon output during the processes when high pressure vm-scability
> test is being taken:
>
> 1) This is pre-commit a2468cc9bfdf, 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 commit a2468cc9bfdf, 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-commit a2468cc9bfdf way 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)
>
> Suggested-by: Chris Li <chrisl@kernel.org>
> Acked-by: Chris Li <chrisl@kernel.org>
> Signed-off-by: Baoquan He <bhe@redhat.com>

Acked-by: Nhat Pham <nphamcs@gmail.com>