On Fri 21-07-17 04:39:48, Matthew Wilcox wrote: > 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?
I have already asked this and didn't get any response. There were other versions of a similar patch without a good clarification... Zhaoyang Huang, please try to formulate the problem you are fixing and why. While it is clear that you add _an_ optimization it is not really clear why we need it and whether it might adversely affect existing workloads. I would rather not touch this code unless there is a strong justification for it. -- Michal Hocko SUSE Labs
