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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Yu Zhao <yuzhao@google.com>
Cc:     Rik van Riel <riel@surriel.com>,
        Dave Chinner <david@fromorbit.com>,
        Jens Axboe <axboe@kernel.dk>,
        SeongJae Park <sj38.park@gmail.com>,
        Linux-MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Manes <ben.manes@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Hillf Danton <hdanton@sina.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Michael Larabel <michael@michaellarabel.com>,
        Michal Hocko <mhocko@suse.com>,
        Michel Lespinasse <michel@lespinasse.org>,
        Roman Gushchin <guro@fb.com>,
        Rong Chen <rong.a.chen@intel.com>,
        SeongJae Park <sjpark@amazon.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Yang Shi <shy828301@gmail.com>, Zi Yan <ziy@nvidia.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        Kernel Page Reclaim v2 <page-reclaim@google.com>,
        Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH v2 00/16] Multigenerational LRU Framework
References: <20210413075155.32652-1-sjpark@amazon.de>
        <3ddd4f8a-8e51-662b-df11-a63a0e75b2bc@kernel.dk>
        <20210413231436.GF63242@dread.disaster.area>
        <f4750f9431bd12b7338a47925de8b17015da51a7.camel@surriel.com>
        <CAOUHufafMcaG8sOS=1YMy2P_6p0R1FzP16bCwpUau7g1-PybBQ@mail.gmail.com>
        <20210414155130.GU3762101@tassilo.jf.intel.com>
        <CAOUHufZ4o4zmW_PyRCXWmBj4OVgVJdC6h1wZsJFMWpGxpzyGdg@mail.gmail.com>
        <20210415030002.GX3762101@tassilo.jf.intel.com>
        <CAOUHufaAz72+0HJu=XPv-6T8rUAdYo_mb5=H+yF3=LfwoufeYg@mail.gmail.com>
Date:   Thu, 15 Apr 2021 16:19:52 +0800
In-Reply-To: <CAOUHufaAz72+0HJu=XPv-6T8rUAdYo_mb5=H+yF3=LfwoufeYg@mail.gmail.com>
        (Yu Zhao's message of "Thu, 15 Apr 2021 01:13:13 -0600")
Message-ID: <87a6q0ot7b.fsf@yhuang6-desk1.ccr.corp.intel.com>
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Yu Zhao <yuzhao@google.com> writes:

> On Wed, Apr 14, 2021 at 9:00 PM Andi Kleen <ak@linux.intel.com> wrote:
>>
>> > We fall back to the rmap when it's obviously not smart to do so. There
>> > is still a lot of room for improvement in this function though, i.e.,
>> > it should be per VMA and NUMA aware.
>>
>> Okay so it's more a question to tune the cross over heuristic. That
>> sounds much easier than replacing everything.
>>
>> Of course long term it might be a problem to maintain too many
>> different ways to do things, but I suppose short term it's a reasonable
>> strategy.
>
> Hi Rik, Ying,
>
> Sorry for being persistent. I want to make sure we are on the same page:
>
> Page table scanning doesn't replace the existing rmap walk. It is
> complementary and only happens when it is likely that most of the
> pages on a system under pressure have been referenced, i.e., out of
> *inactive* pages, by definition of the existing implementation. Under
> such a condition, scanning *active* pages one by one with the rmap is
> likely to cost more than scanning them all at once via page tables.
> When we evict *inactive* pages, we still use the rmap and share a
> common path with the existing code.
>
> Page table scanning falls back to the rmap walk if the page tables of
> a process are apparently sparse, i.e., rss < size of the page tables.
>
> I should have clarified this at the very beginning of the discussion.
> But it has become so natural to me and I assumed we'd all see it this
> way.
>
> Your concern regarding the NUMA optimization is still valid, and it's
> a high priority.

Hi, Yu,

In general, I think it's a good idea to combine the page table scanning
and rmap scanning in the page reclaiming.  For example, if the
working-set is transitioned, we can take advantage of the fast page
table scanning to identify the new working-set quickly.  While we can
fallback to the rmap scanning if the page table scanning doesn't help.

Best Regards,
Huang, Ying