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

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Message-ID: <20200228034248.GE29971@bombadil.infradead.org>
Date:   Thu, 27 Feb 2020 19:42:48 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Huang, Ying" <ying.huang@...el.com>
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

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)?
If so, can you quantify the benefit at all?