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

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Date:   Mon, 19 Mar 2018 09:53:55 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Li RongQing <lirongqing@...du.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        cgroups@...r.kernel.org, hannes@...xchg.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: 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.

[1] http://lkml.kernel.org/r/20180119132544.19569-2-aryabinin@virtuozzo.com

> 
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
>  mm/memcontrol.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 670e99b68aa6..8910d9e8e908 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2480,7 +2480,7 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
>  		if (!ret)
>  			break;
>  
> -		if (!try_to_free_mem_cgroup_pages(memcg, 1,
> +		if (!try_to_free_mem_cgroup_pages(memcg, 1024,
>  					GFP_KERNEL, !memsw)) {
>  			ret = -EBUSY;
>  			break;
> @@ -2610,7 +2610,7 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
>  		if (signal_pending(current))
>  			return -EINTR;
>  
> -		progress = try_to_free_mem_cgroup_pages(memcg, 1,
> +		progress = try_to_free_mem_cgroup_pages(memcg, 1024,
>  							GFP_KERNEL, true);
>  		if (!progress) {
>  			nr_retries--;
> -- 
> 2.11.0

-- 
Michal Hocko
SUSE Labs