linux-kernel - Re: [PATCH] zsmalloc: consider ZS_ALMOST

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Message-ID: <20150710015828.GA692@swordfish>
Date:	Fri, 10 Jul 2015 10:58:28 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] zsmalloc: consider ZS_ALMOST_FULL as migrate source

On (07/10/15 10:32), Minchan Kim wrote:
> There is no reason to prevent select ZS_ALMOST_FULL as migration
> source if we cannot find source from ZS_ALMOST_EMPTY.
> 
> With this patch, zs_can_compact will return more exact result.
> 

wouldn't that be too aggresive?

drainig 'only ZS_ALMOST_EMPTY classes' sounds safer than draining
'ZS_ALMOST_EMPTY and ZS_ALMOST_FULL clases'. you seemed to be worried
that compaction can leave no unused objects in classes, which will
result in zspage allocation happening right after compaction. it looks
like here the chances to cause zspage allocation are even higher. don't
you think so?

> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>  mm/zsmalloc.c |   19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 8c78bcb..7bd7dde 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1687,12 +1687,20 @@ static enum fullness_group putback_zspage(struct zs_pool *pool,
>  static struct page *isolate_source_page(struct size_class *class)
>  {
>  	struct page *page;
> +	int i;
> +	bool found = false;
>  
> -	page = class->fullness_list[ZS_ALMOST_EMPTY];
> -	if (page)
> -		remove_zspage(page, class, ZS_ALMOST_EMPTY);
> +	for (i = ZS_ALMOST_EMPTY; i >= ZS_ALMOST_FULL; i--) {
> +		page = class->fullness_list[i];
> +		if (!page)
> +			continue;
>  
> -	return page;
> +		remove_zspage(page, class, i);
> +		found = true;
> +		break;
> +	}
> +
> +	return found ? page : NULL;
>  }
>  
>  /*
> @@ -1706,9 +1714,6 @@ static unsigned long zs_can_compact(struct size_class *class)
>  {
>  	unsigned long obj_wasted;
>  
> -	if (!zs_stat_get(class, CLASS_ALMOST_EMPTY))
> -		return 0;
> -

well, you asked to add this check like a week or two ago (it's not even
in -next yet) and now you remove it.

>  	obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
>  		zs_stat_get(class, OBJ_USED);
>  

	-ss
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