Date:   Mon, 10 Jan 2022 23:49:32 +0900
From:   Alexey Avramov <hakavlad@inbox.lv>
To:     Yu Zhao <yuzhao@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>, Jens Axboe <axboe@kernel.dk>,
        Jesse Barnes <jsbarnes@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Michael Larabel <Michael@michaellarabel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Will Deacon <will@kernel.org>,
        Ying Huang <ying.huang@intel.com>,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        page-reclaim@google.com, x86@kernel.org, hakavlad@gmail.com
Subject: Re: [PATCH v6 0/9] Multigenerational LRU Framework
Message-ID: <20220110234932.6eb6c356@mail.inbox.lv>
In-Reply-To: <20220104202227.2903605-1-yuzhao@google.com>
References: <20220104202227.2903605-1-yuzhao@google.com>
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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