linux-kernel - Re: [PATCH] mm thp: shrink deferred split THPs harder

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Date:   Wed, 2 Oct 2019 10:40:14 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     kirill.shutemov@...ux.intel.com, ktkhai@...tuozzo.com,
        hannes@...xchg.org, hughd@...gle.com, shakeelb@...gle.com,
        rientjes@...gle.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm thp: shrink deferred split THPs harder

On Wed 02-10-19 07:56:50, Yang Shi wrote:
> The deferred split THPs may get accumulated with some workloads, they
> would get shrunk when memory pressure is hit.  Now we use DEFAULT_SEEKS
> to determine how many objects would get scanned then split if possible,
> but actually they are not like other system cache objects, i.e. inode
> cache which would incur extra I/O if over reclaimed, the unmapped pages
> will not be accessed anymore, so we could shrink them more aggressively.
> 
> We could shrink THPs more pro-actively even though memory pressure is not
> hit, however, IMHO waiting for memory pressure is still a good
> compromise and trade-off.  And, we do have simpler ways to shrink these
> objects harder until we have to take other means do pro-actively drain.
> 
> Change shrinker->seeks to 0 to shrink deferred split THPs harder.

Do you have any numbers on the effect of this patch.

Btw. the whole thing is getting more and more complex and I still
believe the approach is just wrong. We are tunning for something that
doesn't really belong to the memory reclaim in the first place IMHO.
 
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Kirill Tkhai <ktkhai@...tuozzo.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Shakeel Butt <shakeelb@...gle.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> ---
>  mm/huge_memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 3b78910..1d6b1f1 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2955,7 +2955,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  static struct shrinker deferred_split_shrinker = {
>  	.count_objects = deferred_split_count,
>  	.scan_objects = deferred_split_scan,
> -	.seeks = DEFAULT_SEEKS,
> +	.seeks = 0,
>  	.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE |
>  		 SHRINKER_NONSLAB,
>  };
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs