Re: [PATCH v6 3/7] zsmalloc: introduce zs_can_compact() function

[Minchan Kim]

On Tue, Jul 07, 2015 at 08:56:57PM +0900, Sergey Senozhatsky wrote:
> This function checks if class compaction will free any pages.
> Rephrasing -- do we have enough unused objects to form at least
> one ZS_EMPTY page and free it. It aborts compaction if class
> compaction will not result in any (further) savings.
> 
> EXAMPLE (this debug output is not part of this patch set):
> 
> -- class size
> -- number of allocated objects
> -- number of used objects
> -- max objects per zspage
> -- pages per zspage
> -- estimated number of pages that will be freed
> 
> [..]
> class-512 objs:544 inuse:540 maxobj-per-zspage:8  pages-per-zspage:1 zspages-to-free:0
>  ... class-512 compaction is useless. break
> class-496 objs:660 inuse:570 maxobj-per-zspage:33 pages-per-zspage:4 zspages-to-free:2
> class-496 objs:627 inuse:570 maxobj-per-zspage:33 pages-per-zspage:4 zspages-to-free:1
> class-496 objs:594 inuse:570 maxobj-per-zspage:33 pages-per-zspage:4 zspages-to-free:0
>  ... class-496 compaction is useless. break
> class-448 objs:657 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:4
> class-448 objs:648 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:3
> class-448 objs:639 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:2
> class-448 objs:630 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:1
> class-448 objs:621 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:0
>  ... class-448 compaction is useless. break
> class-432 objs:728 inuse:685 maxobj-per-zspage:28 pages-per-zspage:3 zspages-to-free:1
> class-432 objs:700 inuse:685 maxobj-per-zspage:28 pages-per-zspage:3 zspages-to-free:0
>  ... class-432 compaction is useless. break
> class-416 objs:819 inuse:705 maxobj-per-zspage:39 pages-per-zspage:4 zspages-to-free:2
> class-416 objs:780 inuse:705 maxobj-per-zspage:39 pages-per-zspage:4 zspages-to-free:1
> class-416 objs:741 inuse:705 maxobj-per-zspage:39 pages-per-zspage:4 zspages-to-free:0
>  ... class-416 compaction is useless. break
> class-400 objs:690 inuse:674 maxobj-per-zspage:10 pages-per-zspage:1 zspages-to-free:1
> class-400 objs:680 inuse:674 maxobj-per-zspage:10 pages-per-zspage:1 zspages-to-free:0
>  ... class-400 compaction is useless. break
> class-384 objs:736 inuse:709 maxobj-per-zspage:32 pages-per-zspage:3 zspages-to-free:0
>  ... class-384 compaction is useless. break
> [..]
> 
> Every "compaction is useless" indicates that we saved CPU cycles.
> 
> class-512 has
> 	544	object allocated
> 	540	objects used
> 	8	objects per-page
> 
> Even if we have a ALMOST_EMPTY zspage, we still don't have enough room to
> migrate all of its objects and free this zspage; so compaction will not
> make a lot of sense, it's better to just leave it as is.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>

A nit below.

> ---
>  mm/zsmalloc.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 036baa8..b7410c1 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1685,6 +1685,28 @@ static struct page *isolate_source_page(struct size_class *class)
>  	return page;
>  }
>  
> +/*
> + *
> + * Based on the number of unused allocated objects calculate
> + * and return the number of pages that we can free.
> + *
> + * Should be called under class->lock.
> + */
> +static unsigned long zs_can_compact(struct size_class *class)
> +{
> +	unsigned long obj_wasted;
> +
> +	if (!zs_stat_get(class, CLASS_ALMOST_EMPTY))
> +		return 0;
> +
> +	obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
> +		zs_stat_get(class, OBJ_USED);
> +
> +	obj_wasted /= get_maxobj_per_zspage(class->size,
> +			class->pages_per_zspage);

Normally, I insert blank line before last return and I look at
other code under mm, it seems to be a common rule.

> +	return obj_wasted * get_pages_per_zspage(class->size);
> +}
> +
>  static unsigned long __zs_compact(struct zs_pool *pool,
>  				struct size_class *class)
>  {
> @@ -1698,6 +1720,9 @@ static unsigned long __zs_compact(struct zs_pool *pool,
>  
>  		BUG_ON(!is_first_page(src_page));
>  
> +		if (!zs_can_compact(class))
> +			break;
> +
>  		cc.index = 0;
>  		cc.s_page = src_page;
>  
> -- 
> 2.4.5
> 

-- 
Kind regards,
Minchan Kim
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