linux-kernel - Re: [PATCH v2 00/16] Multigenerational LRU Framework

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Date:   Thu, 15 Apr 2021 01:13:13 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Rik van Riel <riel@...riel.com>, Ying Huang <ying.huang@...el.com>
Cc:     Dave Chinner <david@...morbit.com>, Jens Axboe <axboe@...nel.dk>,
        SeongJae Park <sj38.park@...il.com>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Benjamin Manes <ben.manes@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Michael Larabel <michael@...haellarabel.com>,
        Michal Hocko <mhocko@...e.com>,
        Michel Lespinasse <michel@...pinasse.org>,
        Roman Gushchin <guro@...com>,
        Rong Chen <rong.a.chen@...el.com>,
        SeongJae Park <sjpark@...zon.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        Kernel Page Reclaim v2 <page-reclaim@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v2 00/16] Multigenerational LRU Framework

On Wed, Apr 14, 2021 at 9:00 PM Andi Kleen <ak@...ux.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.

Thanks.