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From:   "Huang\, Ying" <ying.huang@intel.com>
To:     Andrea Righi <andrea.righi@canonical.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan@kernel.org>,
        Anchal Agarwal <anchalag@amazon.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] mm: swap: use fixed-size readahead during swapoff
References: <20200413111810.GA801367@xps-13>
Date:   Mon, 13 Apr 2020 21:13:33 +0800
In-Reply-To: <20200413111810.GA801367@xps-13> (Andrea Righi's message of "Mon,
        13 Apr 2020 13:18:10 +0200")
Message-ID: <875ze37cle.fsf@yhuang-dev.intel.com>
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Andrea Righi <andrea.righi@canonical.com> writes:

> 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.
>
> 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).

Why do you need to swapoff after resuming?  The swap device isn't used
except hibernation?  I guess the process is,

1) add swap device to VM
2) hibernate
3) resume
4) swapoff

Some pages are swapped out in step 2?  If os, can we just set
/proc/sys/vm/swappiness to 0 to avoid swapping in step 2?

Best Regards,
Huang, Ying