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From: Yu Zhao <yuzhao@google.com>
To: Michel Lespinasse <michel@lespinasse.org>
Cc: Rik van Riel <riel@surriel.com>,
	Ying Huang <ying.huang@intel.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>, 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
Date: Fri, 23 Apr 2021 20:33:17 -0600	[thread overview]
Message-ID: <CAOUHufZY62nDZiPvFH_SuMLN-4Nxhr0HeOo06RWdTcE9QbhhXg@mail.gmail.com> (raw)
In-Reply-To: <20210415095708.GA6874@lespinasse.org>

On Sun, Apr 18, 2021 at 12:48 AM Michel Lespinasse
<michel@lespinasse.org> wrote:
> On Thu, Apr 15, 2021 at 01:13:13AM -0600, Yu Zhao wrote:
> > 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.
>
> Could you expand a bit more as to how page table scanning and rmap
> scanning coexist ? Say, there is some memory pressure and you want to
> identify good candidate pages to recaim. You could scan processes with
> the page table scanning method, or you could scan the lru list through
> the rmap method. How do you mix the two - when you use the lru/rmap
> method, won't you encounter both pages that are mapped in "dense"
> processes where scanning page tables would have been better, and pages
> that are mapped in "sparse" processes where you are happy to be using
> rmap, and even pges that are mapped into both types of processes at
> once ?  Or, can you change the lru/rmap scan so that it will efficiently
> skip over all dense processes when you use it ?

Hi Michel,

Sorry for the late reply. I was out of town and am still catching up on emails.

That's a great question. Currently the page table scanning isn't smart
enough to know where dense regions are. My plan was to improve it
gradually but it seems it couldn't wait because people have major
concerns over this.

At the moment, the page table scanning decides if a process is worthy
by checking its RSS against the size of its page tables. This can only
avoid extremely sparse regions, meaning the page table scanning will
scan regions that ideally should be covered by the rmap, for some
worse case scenarios. My next step is to add a bloom filter so it can
quickly determine dense regions and target them only.

Given what I just said, the rmap is unlikely to encounter dense
regions, and that's why the perf profile shows its cpu usage drops
from ~30% to ~5%.

Now the question is how we build the bloom filter. A simple answer is
to let the rmap do the legwork, i.e., when it encounters dense
regions, add them to the filter. Of course this means we'll have to
use the rmap more than we do now, which is not ideal for some
workloads but necessary to avoid worst case scenarios.

Does it make sense?

  reply	other threads:[~2021-04-24  2:36 UTC|newest]

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-13  6:56 Yu Zhao
2021-04-13  6:56 ` [PATCH v2 01/16] include/linux/memcontrol.h: do not warn in page_memcg_rcu() if !CONFIG_MEMCG Yu Zhao
2021-04-13  6:56 ` [PATCH v2 02/16] include/linux/nodemask.h: define next_memory_node() if !CONFIG_NUMA Yu Zhao
2021-04-13  6:56 ` [PATCH v2 03/16] include/linux/huge_mm.h: define is_huge_zero_pmd() if !CONFIG_TRANSPARENT_HUGEPAGE Yu Zhao
2021-04-13  6:56 ` [PATCH v2 04/16] include/linux/cgroup.h: export cgroup_mutex Yu Zhao
2021-04-13  6:56 ` [PATCH v2 05/16] mm/swap.c: export activate_page() Yu Zhao
2021-04-13  6:56 ` [PATCH v2 06/16] mm, x86: support the access bit on non-leaf PMD entries Yu Zhao
2021-04-13  6:56 ` [PATCH v2 07/16] mm/vmscan.c: refactor shrink_node() Yu Zhao
2021-04-13  6:56 ` [PATCH v2 08/16] mm: multigenerational lru: groundwork Yu Zhao
2021-04-13  6:56 ` [PATCH v2 09/16] mm: multigenerational lru: activation Yu Zhao
2021-04-13  6:56 ` [PATCH v2 10/16] mm: multigenerational lru: mm_struct list Yu Zhao
2021-04-14 14:36   ` Matthew Wilcox
2021-04-13  6:56 ` [PATCH v2 11/16] mm: multigenerational lru: aging Yu Zhao
2021-04-13  6:56 ` [PATCH v2 12/16] mm: multigenerational lru: eviction Yu Zhao
2021-04-13  6:56 ` [PATCH v2 13/16] mm: multigenerational lru: page reclaim Yu Zhao
2021-04-13  6:56 ` [PATCH v2 14/16] mm: multigenerational lru: user interface Yu Zhao
2021-04-13  6:56 ` [PATCH v2 15/16] mm: multigenerational lru: Kconfig Yu Zhao
2021-04-13  6:56 ` [PATCH v2 16/16] mm: multigenerational lru: documentation Yu Zhao
2021-04-13  7:51 ` [PATCH v2 00/16] Multigenerational LRU Framework SeongJae Park
2021-04-13 16:13   ` Jens Axboe
2021-04-13 16:42     ` SeongJae Park
2021-04-13 23:14     ` Dave Chinner
2021-04-14  2:29       ` Rik van Riel
     [not found]         ` <CAOUHufafMcaG8sOS=1YMy2P_6p0R1FzP16bCwpUau7g1-PybBQ@mail.gmail.com>
2021-04-14  6:15           ` Huang, Ying
2021-04-14  7:58             ` Yu Zhao
2021-04-14  8:27               ` Huang, Ying
2021-04-14 13:51                 ` Rik van Riel
2021-04-14 15:56                   ` Andi Kleen
2021-04-14 15:58                   ` [page-reclaim] " Shakeel Butt
2021-04-14 18:45                   ` Yu Zhao
2021-04-14 15:51           ` Andi Kleen
2021-04-14 15:58             ` Rik van Riel
2021-04-14 19:14               ` Yu Zhao
2021-04-14 19:41                 ` Rik van Riel
2021-04-14 20:08                   ` Yu Zhao
2021-04-14 19:04             ` Yu Zhao
2021-04-15  3:00               ` Andi Kleen
2021-04-15  7:13                 ` Yu Zhao
2021-04-15  8:19                   ` Huang, Ying
2021-04-15  9:57                   ` Michel Lespinasse
2021-04-24  2:33                     ` Yu Zhao [this message]
2021-04-24  3:30                       ` Andi Kleen
2021-04-24  4:16                         ` Yu Zhao
2021-04-14  3:40       ` Yu Zhao
2021-04-14  4:50         ` Dave Chinner
2021-04-14  7:16           ` Yu Zhao
2021-04-14 10:00             ` Yu Zhao
2021-04-15  1:36             ` Dave Chinner
2021-04-24 21:21               ` Yu Zhao
2021-04-14 14:43       ` Jens Axboe
2021-04-14 19:42         ` Yu Zhao
2021-04-15  1:21         ` Dave Chinner
2021-04-14 17:43 ` Johannes Weiner
2021-04-27 10:35   ` Yu Zhao
2021-04-29 23:46 ` Konstantin Kharlamov
2021-04-30  6:37   ` Konstantin Kharlamov
2021-04-30 19:31     ` Yu Zhao

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