Re: [RFC][PATCHv3 4/7] zsmalloc: introduce zs_can_compact() function

[Minchan Kim]

On Thu, Jun 18, 2015 at 08:46:41PM +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>
> ---
>  mm/zsmalloc.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 4b6f12e..18e10a4 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1684,6 +1684,28 @@ static struct page *isolate_source_page(struct size_class *class)
>  	return page;
>  }
>  
> +/*
> + * Make sure that we actually can compact this class,
> + * IOW if migration will release at least one szpage.

                                                 zspage,

> + *
> + * Should be called under class->lock

Please comment about return.

> + */
> +static unsigned long zs_can_compact(struct size_class *class)
> +{
> +	/*
> +	 * Calculate how many unused allocated objects we
> +	 * have and see if we can free any zspages. Otherwise,
> +	 * compaction can just move objects back and forth w/o
> +	 * any memory gain.
> +	 */
> +	unsigned long obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
> +		zs_stat_get(class, OBJ_USED);
> +

I want to check one more thing.

We could have lots of ZS_ALMOST_FULL but no ZS_ALMOST_EMPTY.
In this implementation, compaction cannot have a source so
it would better to bail out.
IOW,

      if (!zs_stat_get(class, CLASS_ALMOST_EMPTY))
              return 0;

> +	obj_wasted /= get_maxobj_per_zspage(class->size,
> +			class->pages_per_zspage);
> +	return obj_wasted;
> +}
> +
>  static unsigned long __zs_compact(struct zs_pool *pool,
>  				struct size_class *class)
>  {
> @@ -1697,6 +1719,9 @@ 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;
>  
> -- 
> 2.4.4
> 
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