Re: çå: [PATCH] mm/memcontrol.c: speed up to force empty a memory cgroup

[Michal Hocko]

On Mon 19-03-18 10:00:41, Li,Rongqing wrote:
> 
> 
> > -----éäåä-----
> > åää: Michal Hocko [mailto:mhocko@xxxxxxxxxx]
> > åéæé: 2018å3æ19æ 16:54
> > æää: Li,Rongqing <lirongqing@xxxxxxxxx>
> > æé: linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> > cgroups@xxxxxxxxxxxxxxx; hannes@xxxxxxxxxxx; Andrey Ryabinin
> > <aryabinin@xxxxxxxxxxxxx>
> > äé: Re: [PATCH] mm/memcontrol.c: speed up to force empty a memory
> > cgroup
> > 
> > On Mon 19-03-18 16:29:30, Li RongQing wrote:
> > > mem_cgroup_force_empty() tries to free only 32 (SWAP_CLUSTER_MAX)
> > > pages on each iteration, if a memory cgroup has lots of page cache, it
> > > will take many iterations to empty all page cache, so increase the
> > > reclaimed number per iteration to speed it up. same as in
> > > mem_cgroup_resize_limit()
> > >
> > > a simple test show:
> > >
> > >   $dd if=aaa  of=bbb  bs=1k count=3886080
> > >   $rm -f bbb
> > >   $time echo 100000000 >/cgroup/memory/test/memory.limit_in_bytes
> > >
> > > Before: 0m0.252s ===> after: 0m0.178s
> > 
> > Andrey was proposing something similar [1]. My main objection was that his
> > approach might lead to over-reclaim. Your approach is more conservative
> > because it just increases the batch size. The size is still rather arbitrary. Same
> > as SWAP_CLUSTER_MAX but that one is a commonly used unit of reclaim in
> > the MM code.
> > 
> > I would be really curious about more detailed explanation why having a
> > larger batch yields to a better performance because we are doingg
> > SWAP_CLUSTER_MAX batches at the lower reclaim level anyway.
> > 
> 
> Although SWAP_CLUSTER_MAX is used at the lower level, but the call stack of 
> try_to_free_mem_cgroup_pages is too long, increase the nr_to_reclaim can reduce
> times of calling function[do_try_to_free_pages, shrink_zones, hrink_node ]
> 
> mem_cgroup_resize_limit
> --->try_to_free_mem_cgroup_pages:  .nr_to_reclaim = max(1024,  SWAP_CLUSTER_MAX),
>    ---> do_try_to_free_pages 
>      ---> shrink_zones
>       --->shrink_node
>        ---> shrink_node_memcg
>          ---> shrink_list          <-------loop will happen in this place [times=1024/32]
>            ---> shrink_page_list

Can you actually measure this to be the culprit. Because we should
rethink our call path if it is too complicated/deep to perform well.
Adding arbitrary batch sizes doesn't sound like a good way to go to me.
-- 
Michal Hocko
SUSE Labs