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From: Michal Hocko <mhocko@kernel.org>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Mel Gorman <mgorman@suse.de>,
	David Hildenbrand <david@redhat.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	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>, Hugh Dickins <hughd@google.com>,
	Alexander Duyck <alexander.duyck@gmail.com>
Subject: Re: [RFC 0/3] mm: Discard lazily freed pages when migrating
Date: Wed, 4 Mar 2020 10:58:02 +0100	[thread overview]
Message-ID: <20200304095802.GE16139@dhcp22.suse.cz> (raw)
In-Reply-To: <87ftep4pzy.fsf@yhuang-dev.intel.com>

On Tue 03-03-20 19:49:53, Huang, Ying wrote:
> Michal Hocko <mhocko@kernel.org> writes:
> 
> > On Tue 03-03-20 16:47:54, Huang, Ying wrote:
> >> Michal Hocko <mhocko@kernel.org> writes:
> >> 
> >> > On Tue 03-03-20 09:51:56, Huang, Ying wrote:
> >> >> Mel Gorman <mgorman@suse.de> writes:
> >> >> > On Mon, Mar 02, 2020 at 07:23:12PM +0800, Huang, Ying wrote:
> >> >> >> If some applications cannot tolerate the latency incurred by the memory
> >> >> >> allocation and zeroing.  Then we cannot discard instead of migrate
> >> >> >> always.  While in some situations, less memory pressure can help.  So
> >> >> >> it's better to let the administrator and the application choose the
> >> >> >> right behavior in the specific situation?
> >> >> >> 
> >> >> >
> >> >> > Is there an application you have in mind that benefits from discarding
> >> >> > MADV_FREE pages instead of migrating them?
> >> >> >
> >> >> > Allowing the administrator or application to tune this would be very
> >> >> > problematic. An application would require an update to the system call
> >> >> > to take advantage of it and then detect if the running kernel supports
> >> >> > it. An administrator would have to detect that MADV_FREE pages are being
> >> >> > prematurely discarded leading to a slowdown and that is hard to detect.
> >> >> > It could be inferred from monitoring compaction stats and checking
> >> >> > if compaction activity is correlated with higher minor faults in the
> >> >> > target application. Proving the correlation would require using the perf
> >> >> > software event PERF_COUNT_SW_PAGE_FAULTS_MIN and matching the addresses
> >> >> > to MADV_FREE regions that were freed prematurely. That is not an obvious
> >> >> > debugging step to take when an application detects latency spikes.
> >> >> >
> >> >> > Now, you could add a counter specifically for MADV_FREE pages freed for
> >> >> > reasons other than memory pressure and hope the administrator knows about
> >> >> > the counter and what it means. That type of knowledge could take a long
> >> >> > time to spread so it's really very important that there is evidence of
> >> >> > an application that suffers due to the current MADV_FREE and migration
> >> >> > behaviour.
> >> >> 
> >> >> OK.  I understand that this patchset isn't a universal win, so we need
> >> >> some way to justify it.  I will try to find some application for that.
> >> >> 
> >> >> Another thought, as proposed by David Hildenbrand, it's may be a
> >> >> universal win to discard clean MADV_FREE pages when migrating if there are
> >> >> already memory pressure on the target node.  For example, if the free
> >> >> memory on the target node is lower than high watermark?
> >> >
> >> > This is already happening because if the target node is short on memory
> >> > it will start to reclaim and if MADV_FREE pages are at the tail of
> >> > inactive file LRU list then they will be dropped. Please note how that
> >> > follows proper aging and doesn't introduce any special casing. Really
> >> > MADV_FREE is an inactive cache for anonymous memory and we treat it like
> >> > inactive page cache. This is not carved in stone of course but it really
> >> > requires very good justification to change.
> >> 
> >> If my understanding were correct, the newly migrated clean MADV_FREE
> >> pages will be put at the head of inactive file LRU list instead of the
> >> tail.  So it's possible that some useful file cache pages will be
> >> reclaimed.
> >
> > This is the case also when you migrate other pages, right? We simply
> > cannot preserve the aging.
> 
> So you consider the priority of the clean MADV_FREE pages is same as
> that of page cache pages?

This is how MADV_FREE has been implemented, yes. See f7ad2a6cb9f7 ("mm:
move MADV_FREE pages into LRU_INACTIVE_FILE list") for the
justification.

> Because the penalty difference is so large, I
> think it may be a good idea to always put clean MADV_FREE pages at the
> tail of the inactive file LRU list?

You are again making assumptions without giving any actual real
examples. Reconstructing MADV_FREE pages cost can differ a lot. This
really depends on the specific usecase. Moving pages to the tail of LRU
would make them the primary candidate for the reclaim with a strange
LIFO semantic. Adding them to the head might be not the universal win
but it will at least provide a reasonable FIFO semantic. I also find
it much more easier to reason about MADV_FREE as an inactive cache.
-- 
Michal Hocko
SUSE Labs


  reply	other threads:[~2020-03-04  9:58 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 [this message]
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
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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