linux-kernel - Re: [RFC PATCH 3/5] mm/page_alloc: stop instantly reusing freed page

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Date:   Thu, 13 Oct 2016 12:59:14 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [RFC PATCH 3/5] mm/page_alloc: stop instantly reusing freed page

On 10/13/2016 10:08 AM, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
>
> Allocation/free pattern is usually sequantial. If they are freed to
> the buddy list, they can be coalesced. However, we first keep these freed
> pages at the pcp list and try to reuse them until threshold is reached
> so we don't have enough chance to get a high order freepage. This reusing
> would provide us some performance advantages since we don't need to
> get the zone lock and we don't pay the cost to check buddy merging.
> But, less fragmentation and more high order freepage would compensate
> this overhead in other ways. First, we would trigger less direct
> compaction which has high overhead. And, there are usecases that uses
> high order page to boost their performance.
>
> Instantly resuing freed page seems to provide us computational benefit
> but the other affects more precious things like as I/O performance and
> memory consumption so I think that it's a good idea to weight
> later advantage more.

Again, there's also cache hotness to consider. And whether the 
sequential pattern is still real on a system with higher uptime. Should 
be possible to evaluate with tracepoints?