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

[Michal Hocko <mhocko@kernel.org>]

On Fri 23-03-18 02:58:36, Li,Rongqing wrote:
> 
> 
> > -----邮件原件-----
> > 发件人: linux-kernel-owner@vger.kernel.org
> > [mailto:linux-kernel-owner@vger.kernel.org] 代表 Li,Rongqing
> > 发送时间: 2018年3月19日 18:52
> > 收件人: Michal Hocko <mhocko@kernel.org>
> > 抄送: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> > cgroups@vger.kernel.org; hannes@cmpxchg.org; Andrey Ryabinin
> > <aryabinin@virtuozzo.com>
> > 主题: 答复: 答复: [PATCH] mm/memcontrol.c: speed up to force empty a
> > memory cgroup
> > 
> > 
> > 
> > > -----邮件原件-----
> > > 发件人: Michal Hocko [mailto:mhocko@kernel.org]
> > > 发送时间: 2018年3月19日 18:38
> > > 收件人: Li,Rongqing <lirongqing@baidu.com>
> > > 抄送: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> > > cgroups@vger.kernel.org; hannes@cmpxchg.org; Andrey Ryabinin
> > > <aryabinin@virtuozzo.com>
> > > 主题: Re: 答复: [PATCH] mm/memcontrol.c: speed up to force empty a
> > memory
> > > cgroup
> > >
> > > On Mon 19-03-18 10:00:41, Li,Rongqing wrote:
> > > >
> > > >
> > > > > -----邮件原件-----
> > > > > 发件人: Michal Hocko [mailto:mhocko@kernel.org]
> > > > > 发送时间: 2018年3月19日 16:54
> > > > > 收件人: Li,Rongqing <lirongqing@baidu.com>
> > > > > 抄送: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> > > > > cgroups@vger.kernel.org; hannes@cmpxchg.org; Andrey Ryabinin
> > > > > <aryabinin@virtuozzo.com>
> > > > > 主题: 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.
> > 
> > Ok, I will try
> > 
> http://pasted.co/4edbcfff
> 
> This is result from ftrace graph, it maybe prove that the deep call
> path leads to low performance.

Does it? Let's have a look at the condensed output:
  6)               |    try_to_free_mem_cgroup_pages() {
  6)               |      mem_cgroup_select_victim_node() {
  6)   0.320 us    |        mem_cgroup_node_nr_lru_pages();
  6)   0.151 us    |        mem_cgroup_node_nr_lru_pages();
  6)   2.190 us    |      }
  6)               |      do_try_to_free_pages() {
  6)               |        shrink_node() {
  6)               |          shrink_node_memcg() {
  6)               |            shrink_inactive_list() {
  6) + 23.131 us   |              shrink_page_list();
  6) + 33.960 us   |            }
  6) + 39.203 us   |          }
  6)               |          shrink_slab() {
  6) + 72.955 us   |          }
  6) ! 116.529 us  |        }
  6)               |        shrink_node() {
  6)   0.050 us    |          mem_cgroup_iter();
  6)   0.035 us    |          mem_cgroup_low();
  6)               |          shrink_node_memcg() {
  6)   3.955 us    |          }
  6)               |          shrink_slab() {
  6) + 54.296 us   |          }
  6) + 61.502 us   |        }
  6) ! 185.020 us  |      }
  6) ! 188.165 us  |    }

try_to_free_mem_cgroup_pages is the full memcg reclaim path taking
188,165 us. The pure reclaim path is shrink_node and that took 116+61 = 177 us.
So we have 11us spent on the way. Is this really making such a difference?
How does the profile look when we do larger batches?

> And when increase reclaiming page in try_to_free_mem_cgroup_pages, it
> can reduce calling of shrink_slab, which save times, in my cases, page
> caches occupy most memory, slab is little, but shrink_slab will be
> called everytime

OK, that makes more sense! shrink_slab is clearly visible here. It is
more expensive than the page reclaim. This is something to look into.

Thanks!
-- 
Michal Hocko
SUSE Labs