Re: [PATCH] mm:workingset use real time to judge activity of the file page

From: Zhaoyang Huang <huangzhaoyang@gmail.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Pavel Tatashin <pasha.tatashin@oracle.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	David Rientjes <rientjes@google.com>,
	Zhaoyang Huang <zhaoyang.huang@unisoc.com>,
	Roman Gushchin <guro@fb.com>, Jeff Layton <jlayton@redhat.com>,
	Matthew Wilcox <mawilcox@microsoft.com>,
	"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm:workingset use real time to judge activity of the file page
Date: Fri, 5 Apr 2019 07:23:46 +0800	[thread overview]
Message-ID: <CAGWkznHeGeiHWSF-gPmSW=AWQcEybHroOuP4CSbW4rKq12LtNw@mail.gmail.com> (raw)
In-Reply-To: <20190404163914.GA4229@cmpxchg.org>

On Fri, Apr 5, 2019 at 12:39 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Thu, Apr 04, 2019 at 11:30:17AM +0800, Zhaoyang Huang wrote:
> > From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> >
> > In previous implementation, the number of refault pages is used
> > for judging the refault period of each page, which is not precised as
> > eviction of other files will be affect a lot on current cache.
> > We introduce the timestamp into the workingset's entry and refault ratio
> > to measure the file page's activity. It helps to decrease the affection
> > of other files(average refault ratio can reflect the view of whole system
> > 's memory).
>
> I don't understand what exactly you're saying here, can you please
> elaborate?
>
> The reason it's using distances instead of absolute time is because
> the ordering of the LRU is relative and not based on absolute time.
>
> E.g. if a page is accessed every 500ms, it depends on all other pages
> to determine whether this page is at the head or the tail of the LRU.
>
> So when you refault, in order to determine the relative position of
> the refaulted page in the LRU, you have to compare it to how fast that
> LRU is moving. The absolute refault time, or the average time between
> refaults, is not comparable to what's already in memory.
How do you know how long time did these pages' dropping taken.Actruly,
a quick dropping of large mount of pages will be wrongly deemed as
slow dropping instead of the exact hard situation.That is to say, 100
pages per million second or per second have same impaction on
calculating the refault distance, which may cause less protection on
this page cache for former scenario and introduce page thrashing.
especially when global reclaim, a round of kswapd reclaiming that
waked up by a high order allocation or large number of single page
allocations may cause such things as all pages within the node are
counted in the same lru. This commit can decreasing above things by
comparing refault time of single page with avg_refault_time =
delta_lru_reclaimed_pages/ avg_refault_retio (refault_ratio =
lru->inactive_ages / time).