Re: [PATCH] mm: swap: introduce fixed-size readahead policy

[Andrea Righi <andrea.righi@canonical.com>]

On Mon, Apr 13, 2020 at 09:24:14AM +0800, Huang, Ying wrote:
> Andrea Righi <andrea.righi@canonical.com> writes:
> 
> > Introduce a new fixed-size swap-in readahead policy that can be selected
> > at run-time.
> >
> > The global swap-in readahead policy takes in account the previous access
> > patterns, using a scaling heuristic to determine the optimal readahead
> > chunk dynamically.
> >
> > This works pretty well in most cases, but like any heuristic there are
> > specific cases when this approach is not ideal, for example the swapoff
> > scenario.
> >
> > During swapoff we just want to load back into memory all the swapped-out
> > pages and for this specific use case a fixed-size readahead is more
> > efficient.
> >
> > This patch introduces a new sysfs interface
> > (/sys/kernel/mm/swap/swap_ra_policy) that can be set as following:
> >
> >  - 0: current scaling swap-in readahead policy (default)
> >  - 1: fixed-size readahead policy (size is determined by
> >       vm.page-cluster)
> >
> > The specific use case this patch is addressing is to improve swapoff
> > performance when a VM has been hibernated, resumed and all memory needs
> > to be forced back to RAM by disabling swap (see the test case below).
> 
> I don't know whether the use case is important or not.  Usually the
> performance of swapoff doesn't matter.  Maybe you can provide some
> information on this.

In general yes, I agree, swapoff performance is not that important.

This particular optimization has been tested a lot in the Ubuntu AWS
kernel and it seems to speed up consistently the resume of hibernated
EC2 instances (see also [1]).

So we were considering to upstream this, if we find a way to make it
more suitable for mainline inclusion.

[1] https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/Hibernate.html

> 
> Even if it's important, I don't think we really need to add another ABI.
> You can just optimize the swapoff performance inside kernel.  And we may
> not even need to add a new policy, just use cluster readahead and
> increase swapin_readahead_hits in swapoff if cluster readahead is used.

OK, instead of adding a new ABI I can make this optimization directly in
the kernel, so that when swapoff runs it can automatically use the
fixed-size cluster readahead and continue using the default readahead
for everything else.

I'll play a bit with this and will send a new patch.

Thanks!
-Andrea