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Sun, 12 Oct 2025 13:41:06 -0700 (PDT) MIME-Version: 1.0 References: <20251011081624.224202-1-bhe@redhat.com> <20251011081624.224202-3-bhe@redhat.com> In-Reply-To: <20251011081624.224202-3-bhe@redhat.com> From: Barry Song <21cnbao@gmail.com> Date: Mon, 13 Oct 2025 04:40:54 +0800 X-Gm-Features: AS18NWDMFFIfVuRNn3FKOPIbqxi90GD7_EunJ-Z8Wurap9DCZm2jWlGrRyASNgk Message-ID: Subject: Re: [PATCH v4 mm-new 2/2] mm/swap: select swap device with default priority round robin To: Baoquan He Cc: linux-mm@kvack.org, akpm@linux-foundation.org, chrisl@kernel.org, kasong@tencent.com, youngjun.park@lge.com, aaron.lu@intel.com, shikemeng@huaweicloud.com, nphamcs@gmail.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 4ED17A0007 X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: gytn5bmo19ie1e4czqytg817zhkrbqsg X-HE-Tag: 1760301667-421703 X-HE-Meta: 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 7JAU74p+ hJ9ceMjRnhYbM91y5MQ4tXgQWvSYVexNqGhGhkw5/+wDpIoCdYvLJp/Tx0Py2yElbZ/aGz9iCAW0pz2j5L+BscDpqQtX98NX79S+rVWlZegBAlBKW43MJhyj/eg+nZnq0tV4VXW8l4cHve7hggUB3KE3c0B1BWTBHFSaV1Sk4CCCfqyEE6xNGFLi/PS4qK6S1W+KMkDcLsowTuxv3cTqujAYwZbDvGGewp1NyoIq5sGpXIY4Eehfedx37++ro/MqKI54kRSToWzFmAJPlOtk3zFirOK3VVEHgxH1W7TDLjkrorSThUdPuNtpV0Ff7oGVuDSlH2nALPsV8cm0a4UPoR+hS0f3KmtXtFzKE6IMjFGQuguYhWz5cHEIe7/jx0MwQFD9p X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Oct 12, 2025 at 5:14=E2=80=AFAM Baoquan He wrote: > > Swap devices are assumed to have similar accessing speed if no priority > is specified when swapon. 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 processes 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 selec= td > 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, we can see about 18% efficiency promotion relative to > node based way as below. (Surely, the pre-commit a2468cc9bfdf way is > the worst.) > I=E2=80=99m not against the behavior change; but the swapon man page says: " Each swap area has a priority, either high or low. The default priority is low. Within the low-priority areas, newer areas are even lower priority than older areas. " So my question is whether users still assume that newly added swap areas get a lower priority than the older ones? I assume the priority decrement isn=E2=80=99t a stable ABI, so this change = won=E2=80=99t break userspace? Or if someone sets up Linux assuming that a newer swap file will only be used after the older one is full, then this change would break those cases? Thanks Barry