Date: Fri, 21 Jul 2017 04:39:48 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Zhaoyang Huang <huangzhaoyang@gmail.com>
Cc: zhaoyang.huang@spreadtrum.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>, Ingo Molnar <mingo@kernel.org>,
        zijun_hu <zijun_hu@htc.com>, Vlastimil Babka <vbabka@suse.cz>,
        Thomas Garnier <thgarnie@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, zijun_hu@zoho.com
Subject: Re: [PATCH v1] mm/vmalloc: add a node corresponding to
 cached_hole_size
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On Fri, Jul 21, 2017 at 06:01:41PM +0800, Zhaoyang Huang wrote:
> we just record the cached_hole_size now, which will be used when
> the criteria meet both of 'free_vmap_cache == NULL' and 'size <
> cached_hole_size'. However, under above scenario, the search will
> start from the rb_root and then find the node which just in front
> of the cached hole.
> 
> free_vmap_cache miss:
>       vmap_area_root
>           /      \
>        _next     U
>         /  (T1)
>  cached_hole_node
>        /
>      ...   (T2)
>       /
>     first
> 
> vmap_area_list->first->......->cached_hole_node->cached_hole_node.list.next
>                   |-------(T3)-------| | <<< cached_hole_size >>> |
> 
> vmap_area_list->......->cached_hole_node->cached_hole_node.list.next
>                                | <<< cached_hole_size >>> |
> 
> The time cost to search the node now is T = T1 + T2 + T3.
> The commit add a cached_hole_node here to record the one just in front of
> the cached_hole_size, which can help to avoid walking the rb tree and
> the list and make the T = 0;

Yes, but does this matter in practice?  Are there any workloads where
this makes a difference?  If so, how much?