linux-kernel - Re: [PATCH v6 0/9] Multigenerational LRU Framework

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Date:   Mon, 10 Jan 2022 23:49:32 +0900
From:   Alexey Avramov <hakavlad@...ox.lv>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        Ying Huang <ying.huang@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        page-reclaim@...gle.com, x86@...nel.org, hakavlad@...il.com
Subject: Re: [PATCH v6 0/9] Multigenerational LRU Framework

Note that with vm.swappiness=0, the vm.watermark_scale_factor value does
not affect the swap possibility: MGLRU ignores sc->file_is_tiny.

With the classic 2-gen LRU swapping goes well at swappiness=0 and
high vm.watermark_scale_factor, which is expected according to the
documentation:
"At 0, the kernel will not initiate swap until the amount of free and
file-backed pages is less than the high watermark in a zone." [1]

With vm.swappiness=0, no swap occurs with any vm.watermark_scale_factor
value with MGLRU.

I used to see in practice (with MGLRU v3) the impossibility of swapping 
when vm.swappiness=0 and vm.watermark_scale_factor=1000.

At a minimum, this will require updating the documentation for
vm.swappiness.

BTW, why MGLRU doesn't use something like sc->file_is_tiny?

[1] https://github.com/torvalds/linux/blob/v5.16/Documentation/admin-guide/sysctl/vm.rst#swappiness