linux-kernel - Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

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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