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From: Michal Hocko <mhocko@kernel.org>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: David Hildenbrand <david@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
	Zi Yan <ziy@nvidia.com>, Peter Zijlstra <peterz@infradead.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Minchan Kim <minchan@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Hugh Dickins <hughd@google.com>,
	Alexander Duyck <alexander.duyck@gmail.com>
Subject: Re: [RFC 0/3] mm: Discard lazily freed pages when migrating
Date: Fri, 28 Feb 2020 11:15:17 +0100	[thread overview]
Message-ID: <20200228101517.GM3771@dhcp22.suse.cz> (raw)
In-Reply-To: <20200228095048.GK3771@dhcp22.suse.cz>

On Fri 28-02-20 10:50:50, Michal Hocko wrote:
> On Fri 28-02-20 16:55:40, Huang, Ying wrote:
> > David Hildenbrand <david@redhat.com> writes:
> [...]
> > > E.g., free page reporting in QEMU wants to use MADV_FREE. The guest will
> > > report currently free pages to the hypervisor, which will MADV_FREE the
> > > reported memory. As long as there is no memory pressure, there is no
> > > need to actually free the pages. Once the guest reuses such a page, it
> > > could happen that there is still the old page and pulling in in a fresh
> > > (zeroed) page can be avoided.
> > >
> > > AFAIKs, after your change, we would get more pages discarded from our
> > > guest, resulting in more fresh (zeroed) pages having to be pulled in
> > > when a guest touches a reported free page again. But OTOH, page
> > > migration is speed up (avoiding to migrate these pages).
> > 
> > Let's look at this problem in another perspective.  To migrate the
> > MADV_FREE pages of the QEMU process from the node A to the node B, we
> > need to free the original pages in the node A, and (maybe) allocate the
> > same number of pages in the node B.  So the question becomes
> > 
> > - we may need to allocate some pages in the node B
> > - these pages may be accessed by the application or not
> > - we should allocate all these pages in advance or allocate them lazily
> >   when they are accessed.
> > 
> > We thought the common philosophy in Linux kernel is to allocate lazily.
> 
> The common philosophy is to cache as much as possible. And MADV_FREE
> pages are a kind of cache as well. If the target node is short on memory
> then those will be reclaimed as a cache so a pro-active freeing sounds
> counter productive as you do not have any idea whether that cache is
> going to be used in future. In other words you are not going to free a
> clean page cache if you want to use that memory as a migration target

sorry I meant to say that you are not going to clean page cache when
migrating it.

> right? So you should make a clear case about why MADV_FREE cache is less
> important than the clean page cache and ideally have a good
> justification backed by real workloads.

-- 
Michal Hocko
SUSE Labs


  reply	other threads:[~2020-02-28 10:15 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-28  3:38 [RFC 0/3] mm: Discard lazily freed pages when migrating Huang, Ying
2020-02-28  3:38 ` [RFC 1/3] mm, migrate: Check return value of try_to_unmap() Huang, Ying
2020-02-28  3:38 ` [RFC 2/3] mm: Add a new page flag PageLayzyFree() for MADV_FREE Huang, Ying
2020-02-28  6:13   ` David Hildenbrand
2020-02-28  6:47     ` Huang, Ying
2020-03-15  8:18   ` Wei Yang
2020-03-15  8:54     ` Mika Penttilä
2020-03-15 12:22       ` Wei Yang
2020-03-16  1:21         ` Huang, Ying
2020-03-16 22:38           ` Wei Yang
2020-02-28  3:38 ` [RFC 3/3] mm: Discard lazily freed pages when migrating Huang, Ying
2020-02-28  3:42 ` [RFC 0/3] " Matthew Wilcox
2020-02-28  7:25   ` Huang, Ying
2020-02-28  8:22     ` David Hildenbrand
2020-02-28  8:55       ` Huang, Ying
2020-02-28  9:49         ` Mel Gorman
2020-03-02 11:23           ` Huang, Ying
2020-03-02 15:16             ` Mel Gorman
2020-03-03  1:51               ` Huang, Ying
2020-03-03  8:09                 ` Michal Hocko
2020-03-03  8:47                   ` Huang, Ying
2020-03-03  8:58                     ` Michal Hocko
2020-03-03 11:49                       ` Huang, Ying
2020-03-04  9:58                         ` Michal Hocko
2020-03-04 10:56                           ` Mel Gorman
2020-03-05  1:42                             ` Huang, Ying
2020-03-04 11:15                           ` Huang, Ying
2020-03-04 11:26                             ` Michal Hocko
2020-03-05  1:45                               ` Huang, Ying
2020-03-05 10:48                             ` Mel Gorman
2020-03-06  4:05                               ` Huang, Ying
2020-03-09  5:26                               ` Huang, Ying
2020-03-03 13:02                 ` Mel Gorman
2020-03-04  0:33                   ` Huang, Ying
2020-02-28  9:50         ` Michal Hocko
2020-02-28 10:15           ` Michal Hocko [this message]
2020-02-28 13:45           ` Johannes Weiner
2020-03-02 14:12           ` Huang, Ying
2020-03-02 14:23             ` David Hildenbrand
2020-03-03  0:25               ` Huang, Ying
2020-03-02 14:25             ` Michal Hocko
2020-03-03  1:30               ` Huang, Ying
2020-03-03  8:19                 ` Michal Hocko
2020-03-03 11:36                   ` Huang, Ying

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