From: Mel Gorman <mgorman@suse.de>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: David Hildenbrand <david@redhat.com>,
Michal Hocko <mhocko@kernel.org>,
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: Tue, 3 Mar 2020 13:02:41 +0000 [thread overview]
Message-ID: <20200303130241.GE3772@suse.de> (raw)
In-Reply-To: <87zhcy5hoj.fsf@yhuang-dev.intel.com>
On Tue, Mar 03, 2020 at 09:51:56AM +0800, 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?
>
That is an extremely specific corner case that is not likely to occur.
NUMA balancing is not going to migrate a MADV_FREE page under these
circumstances as a write cancels MADV_FREE is read attempt will probably
fail to allocate a destination page in alloc_misplaced_dst_page so the
data gets lost instead of remaining remote. sys_movepages is a possibility
but the circumstances of an application delibertly trying to migrate to
a loaded node is low. Compaction never migrates cross-node so the state
of a remote node under pressure do not matter.
Once again, there needs to be a reasonable use case to be able to
meaningfully balance between the benefits and risks of changing the
MADV_FREE semantics.
--
Mel Gorman
SUSE Labs
next prev parent reply other threads:[~2020-03-03 13:02 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 [this message]
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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