Re: [RFC PATCH 3/3] mm/zsmalloc: change ZS_MAX_PAGES_PER_ZSPAGE

[Sergey Senozhatsky]

On (02/18/16 13:41), Sergey Senozhatsky wrote:
> I think we better switch to different logic here -- specify how many ->huge
> classes we want to have, and let zsmalloc to calculate ZS_MAX_PAGES_PER_ZSPAGE.
> 
> 
> For example, if we want to have 20 ->huge classes, the 'smallest' (or the last)
							^^^ largest
> non-huge class will have CLASS_SIZE * SIZE_CLASS_DELTA of spare (wasted) space,
> so  PAGE_SIZE / (CLASS_SIZE * SIZE_CLASS_DELTA)  will give us the number of pages
> we need to form into a zspage to make it the last huge class.
						    ^^ 'non-huge'... what is the
						    correct english word I'm looking
						    for...

	-ss

> setting ZS_MIN_HUGE_CLASSES_NUM to 32 gives us (on x86_64, PAGE_SHIFT 12) ->huge
> class size range of [3648, 4096]. so all objects smaller than 3648 will not waste
> an entire zspage (and order-0 page), but will share the page with another objects
> of that size.
> 
> 
> something like this:
> 
> ---
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 0c9f117..d5252d1 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -73,12 +73,6 @@
>   */
>  #define ZS_ALIGN               8
>  
> -/*
> - * A single 'zspage' is composed of up ZS_MAX_PAGES_PER_ZSPAGE discontiguous
> - * 0-order (single) pages.
> - */
> -#define ZS_MAX_PAGES_PER_ZSPAGE        6
> -
>  #define ZS_HANDLE_SIZE (sizeof(unsigned long))
>  
>  /*
> @@ -149,6 +143,21 @@
>  #define ZS_SIZE_CLASS_DELTA    (PAGE_SIZE >> 8)
>  
>  /*
> + * We want to have at least this number of ->huge classes.
> + */
> +#define ZS_MIN_HUGE_CLASSES_NUM        32
> +/*
> + * A single 'zspage' is composed of up ZS_MAX_PAGES_PER_ZSPAGE discontiguous
> + * 0-order (single) pages.
> + *
> + * The smallest huge class will have CLASS_SIZE * SIZE_CLASS_DELTA of
> + * wasted space, calculate how many pages we need to fit a CLASS_SIZE
> + * object there and, thus, to save a additional zspage.
> + */
> +#define ZS_MAX_PAGES_PER_ZSPAGE        \
> +       (PAGE_SIZE / (ZS_MIN_HUGE_CLASSES_NUM * ZS_SIZE_CLASS_DELTA))
> +
> +/*
>   * We do not maintain any list for completely empty or full pages
>   */
>  enum fullness_group {
>