Re: [RFC][PATCHv2 3/8] zsmalloc: lower ZS_ALMOST_FULL waterline

[Minchan Kim]

On Fri, Jun 05, 2015 at 09:03:53PM +0900, Sergey Senozhatsky wrote:
> get_fullness_group() considers 3/4 full pages as almost empty.
> That, unfortunately, marks as ALMOST_EMPTY pages that we would
> probably like to keep in ALMOST_FULL lists.
> 
> ALMOST_EMPTY:
> [..]
>   inuse: 3 max_objects: 4
>   inuse: 5 max_objects: 7
>   inuse: 5 max_objects: 7
>   inuse: 2 max_objects: 3
> [..]
> 
> For "inuse: 5 max_objexts: 7" ALMOST_EMPTY page, for example,
> it'll take 2 obj_malloc to make the page FULL and 5 obj_free to
> make it EMPTY. Compaction selects ALMOST_EMPTY pages as source
> pages, which can result in extra object moves.
> 
> In other words, from compaction point of view, it makes more
> sense to fill this page, rather than drain it.
> 
> Decrease ALMOST_FULL waterline to 2/3 of max capacity; which is,
> of course, still imperfect, but can shorten compaction
> execution time.

However, at worst case, once compaction is done, it could remain
33% fragment space while it can remain 25% fragment space in current.
Maybe 25% wouldn't enough so we might need to scan ZS_ALMOST_FULL as
source in future. Anyway, compaction is really slow path now so
I prefer saving memory space by reduce internal fragmentation to
performance caused more copy of objects.

> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> ---
>  mm/zsmalloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index cd37bda..b94e281 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -198,7 +198,7 @@ static int zs_size_classes;
>   *
>   * (see: fix_fullness_group())
>   */
> -static const int fullness_threshold_frac = 4;
> +static const int fullness_threshold_frac = 3;
>  
>  struct size_class {
>  	/*
> @@ -633,7 +633,7 @@ static enum fullness_group get_fullness_group(struct page *page)
>  		fg = ZS_EMPTY;
>  	else if (inuse == max_objects)
>  		fg = ZS_FULL;
> -	else if (inuse <= 3 * max_objects / fullness_threshold_frac)
> +	else if (inuse <= 2 * max_objects / fullness_threshold_frac)
>  		fg = ZS_ALMOST_EMPTY;
>  	else
>  		fg = ZS_ALMOST_FULL;
> -- 
> 2.4.2.387.gf86f31a
> 

-- 
Kind regards,
Minchan Kim
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