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

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Date:   Mon, 13 Apr 2020 09:24:14 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Andrea Righi <andrea.righi@...onical.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Anchal Agarwal <anchalag@...zon.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: swap: introduce fixed-size readahead policy

Andrea Righi <andrea.righi@...onical.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.

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.

Best Regards,
Huang, Ying