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Message-ID: <87a7538977.fsf@yhuang-dev.intel.com>
Date: Fri, 28 Feb 2020 15:25:32 +0800
From: "Huang\, Ying" <ying.huang@...el.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>,
David Hildenbrand <david@...hat.com>,
"Mel Gorman" <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
Zi Yan <ziy@...dia.com>, Michal Hocko <mhocko@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"Minchan Kim" <minchan@...nel.org>,
Johannes Weiner <hannes@...xchg.org>,
Hugh Dickins <hughd@...gle.com>
Subject: Re: [RFC 0/3] mm: Discard lazily freed pages when migrating
Hi, Matthew,
Matthew Wilcox <willy@...radead.org> writes:
> On Fri, Feb 28, 2020 at 11:38:16AM +0800, Huang, Ying wrote:
>> MADV_FREE is a lazy free mechanism in Linux. According to the manpage
>> of mavise(2), the semantics of MADV_FREE is,
>>
>> The application no longer requires the pages in the range specified
>> by addr and len. The kernel can thus free these pages, but the
>> freeing could be delayed until memory pressure occurs. ...
>>
>> Originally, the pages freed lazily by MADV_FREE will only be freed
>> really by page reclaiming when there is memory pressure or when
>> unmapping the address range. In addition to that, there's another
>> opportunity to free these pages really, when we try to migrate them.
>>
>> The main value to do that is to avoid to create the new memory
>> pressure immediately if possible. Instead, even if the pages are
>> required again, they will be allocated gradually on demand. That is,
>> the memory will be allocated lazily when necessary. This follows the
>> common philosophy in the Linux kernel, allocate resources lazily on
>> demand.
>
> Do you have an example program which does this (and so benefits)?
Sorry, what do you mean exactly for "this" here? Call
madvise(,,MADV_FREE)? Or migrate pages?
> If so, can you quantify the benefit at all?
The question is what is the right workload? For example, I can build a
scenario as below to show benefit.
- run program A in node 0 with many lazily freed pages
- run program B in node 1, so that the free memory on node 1 is low
- migrate the program A from node 0 to node 1, so that the program B is
influenced by the memory pressure created by migrating lazily freed
pages.
Best Regards,
Huang, Ying
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