Re: [PATCH v1] mm/vmalloc: add a node corresponding to cached_hole_size

[Matthew Wilcox]

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?