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Subject答复: 答复: 答复: [PATCH] mm/memcontrol .c: speed up to force empty a memory cgroup
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> -----邮件原件-----
> 发件人: Michal Hocko [mailto:mhocko@kernel.org]
> 发送时间: 2018年3月23日 18:09
> 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: 答复: [PATCH] mm/memcontrol.c: speed up to force empty
> a memory cgroup
>
> 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.
>

shrink_slab() {
6) 0.175 us | down_read_trylock();
6) | super_cache_count() {
6) 0.642 us | list_lru_count_one();
6) 0.587 us | list_lru_count_one();
6) 3.740 us | }
6) | super_cache_count() {
6) 0.625 us | list_lru_count_one();
6) 0.485 us | list_lru_count_one();
6) 2.145 us | }
6) | super_cache_count() {
6) 0.333 us | list_lru_count_one();
6) 0.334 us | list_lru_count_one();
6) 2.109 us | }
6) | super_cache_count() {
6) 0.062 us | list_lru_count_one();
6) 0.188 us | list_lru_count_one();
6) 1.216 us | }
6) | super_cache_count() {
6) 0.217 us | list_lru_count_one();
6) 0.056 us | list_lru_count_one();
6) 1.282 us | }
6) | super_cache_count() {
6) 0.204 us | list_lru_count_one();
6) 0.205 us | list_lru_count_one();
6) 1.237 us | }
6) | super_cache_count() {
6) 0.596 us | list_lru_count_one();
6) 0.493 us | list_lru_count_one();
6) 2.140 us | }
6) | super_cache_count() {
6) 0.130 us | list_lru_count_one();
6) 0.056 us | list_lru_count_one();
6) 1.260 us | }
6) | super_cache_count() {
6) 0.385 us | list_lru_count_one();
6) 0.054 us | list_lru_count_one();
6) 1.186 us | }
6) | super_cache_count() {
6) 0.304 us | list_lru_count_one();
6) 0.286 us | list_lru_count_one();
6) 1.550 us | }
6) | super_cache_count() {
6) 0.230 us | list_lru_count_one();
6) 0.128 us | list_lru_count_one();
6) 1.408 us | }
6) | super_cache_count() {
6) 0.392 us | list_lru_count_one();
6) 0.132 us | list_lru_count_one();
6) 1.694 us | }
6) | super_cache_count() {
6) 0.257 us | list_lru_count_one();
6) 0.258 us | list_lru_count_one();
6) 1.510 us | }
6) | super_cache_count() {
6) 0.132 us | list_lru_count_one();
6) 0.132 us | list_lru_count_one();
6) 1.361 us | }
6) | super_cache_count() {
6) 0.130 us | list_lru_count_one();
6) 0.130 us | list_lru_count_one();
6) 1.246 us | }
6) | count_shadow_nodes() {
6) 0.203 us | list_lru_count_one();
6) 0.042 us | mem_cgroup_node_nr_lru_pages();
6) 1.131 us | }
6) | super_cache_count() {
6) 0.202 us | list_lru_count_one();
6) 0.056 us | list_lru_count_one();
6) 1.115 us | }
6) | super_cache_count() {
6) 0.055 us | list_lru_count_one();
6) 0.107 us | list_lru_count_one();
6) 0.958 us | }
6) | super_cache_count() {
6) 0.147 us | list_lru_count_one();
6) 0.150 us | list_lru_count_one();
6) 1.474 us | }
6) | super_cache_count() {
6) 0.491 us | list_lru_count_one();
6) 0.485 us | list_lru_count_one();
6) 2.569 us | }
6) | super_cache_count() {
6) 0.605 us | list_lru_count_one();
6) 0.590 us | list_lru_count_one();
6) 2.136 us | }
6) | super_cache_count() {
6) 0.572 us | list_lru_count_one();
6) 0.418 us | list_lru_count_one();
6) 1.914 us | }
6) | super_cache_count() {
6) 0.428 us | list_lru_count_one();
6) 0.358 us | list_lru_count_one();
6) 2.073 us | } /* super_cache_count */
6) | super_cache_count() {
6) 0.422 us | list_lru_count_one();
6) 0.433 us | list_lru_count_one();
6) 1.604 us | }
6) | super_cache_count() {
6) 0.532 us | list_lru_count_one();
6) 0.280 us | list_lru_count_one();
6) 1.523 us | }
6) | super_cache_count() {
6) 0.422 us | list_lru_count_one();
6) 0.574 us | list_lru_count_one();
6) 1.554 us | }
6) | super_cache_count() {
6) 0.565 us | list_lru_count_one();
6) 0.587 us | list_lru_count_one();
6) 1.878 us | }
6) | super_cache_count() {
6) 0.563 us | list_lru_count_one();
6) 0.558 us | list_lru_count_one();
6) 1.949 us | }
6) | super_cache_count() {
6) 0.468 us | list_lru_count_one();
6) 0.476 us | list_lru_count_one();
6) 2.149 us | }
6) | super_cache_count() {
6) 0.443 us | list_lru_count_one();
6) 0.483 us | list_lru_count_one();
6) 2.283 us | }
6) | super_cache_count() {
6) 0.332 us | list_lru_count_one();
6) 0.228 us | list_lru_count_one();
6) 1.307 us | }
6) | super_cache_count() {
6) 0.532 us | list_lru_count_one();
6) 0.367 us | list_lru_count_one();
6) 1.956 us | }
6) 0.036 us | up_read();
6) 0.038 us | _cond_resched();
6) + 72.955 us | }

shrink_slab does not reclaim any memory, but take lots of time to count lru

maybe we can use the returning of shrink_slab to control if next shrink_slab should be called?


Or define a slight list_lru_empty to check if sb->s_dentry_lru is empty before calling list_lru_shrink_count, like below

diff --git a/fs/super.c b/fs/super.c
index 672538ca9831..954c22338833 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -130,8 +130,10 @@ static unsigned long super_cache_count(struct shrinker *shrink,
if (sb->s_op && sb->s_op->nr_cached_objects)
total_objects = sb->s_op->nr_cached_objects(sb, sc);

- total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
- total_objects += list_lru_shrink_count(&sb->s_inode_lru, sc);
+ if (!list_lru_empty(sb->s_dentry_lru))
+ total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
+ if (!list_lru_empty(sb->s_inode_lru))
+ total_objects += list_lru_shrink_count(&sb->s_inode_lru, sc);

total_objects = vfs_pressure_ratio(total_objects);
return total_objects;
-RongQing


> Thanks!
> --
> Michal Hocko
> SUSE Labs
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