Re: [PATCH v3 0/2] mm/swap: skip empty clusters in the swapoff scan

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 7 Aug 2026 16:42:22 +0800 Baoquan He <baoquan.he@linux.dev> wrote:

> > I have no measured times yet, since that case has to be set up on purpose.
> > What I did is the arithmetic for the case that skips best,
> > For example 1T of swap with 256M slots with SWAPFILE_CLUSTER = 512
> > and everything free but the far end:
> > 
> >   - today:          256M table reads
> >   - with the skip:  512K counter reads
> 
> 
> Maybe just use time to measure swapoff time consuming, just like below
> as I did on a kvm guest, I guess a bare metal machine with larger system
> ram could be more obvious?
> 
> root@fedora:~# free -h
>                total        used        free      shared  buff/cache   available
> Mem:           3.8Gi       181Mi       3.6Gi       924Ki        72Mi       3.7Gi
> Swap:          2.0Gi        18Mi       2.0Gi
> root@fedora:~# time swapoff /dev/vdb
> 
> real	0m0.101s
> user	0m0.001s
> sys	0m0.017s
> root@fedora:~# swapon /dev/vdb
> root@fedora:~# time swapoff /dev/vdb
> 
> real	0m0.014s
> user	0m0.003s
> sys	0m0.001s
> 
> Not sure if Andrew is asking for this.

I think it's helpful to include such info.  The audience for changelogs
is more than swap developers!  It's also an MM maintainer and -stable
maintainers and other people who are all wondering "should I backport
this for my users".  Let's give them the means to determine that.