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SubjectRe: [PATCHv2 2/2] zram: drop max_zpage_size and use zs_huge_class_size()
Hi Sergey,

Sorry for being late.
I love this patchset! Just a minor below.

On Tue, Mar 06, 2018 at 04:06:39PM +0900, Sergey Senozhatsky wrote:
> This patch removes ZRAM's enforced "huge object" value and uses
> zsmalloc huge-class watermark instead, which makes more sense.
>
> TEST
> - I used a 1G zram device, LZO compression back-end, original
> data set size was 444MB. Looking at zsmalloc classes stats the
> test ended up to be pretty fair.
>
> BASE ZRAM/ZSMALLOC
> =====================
> zram mm_stat
>
> 498978816 191482495 199831552 0 199831552 15634 0
>
> zsmalloc classes
>
> class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
> ...
> 151 2448 0 0 1240 1240 744 3 0
> 168 2720 0 0 4200 4200 2800 2 0
> 190 3072 0 0 10100 10100 7575 3 0
> 202 3264 0 0 380 380 304 4 0
> 254 4096 0 0 10620 10620 10620 1 0
>
> Total 7 46 106982 106187 48787 0
>
> PATCHED ZRAM/ZSMALLOC
> =====================
>
> zram mm_stat
>
> 498978816 182579184 194248704 0 194248704 15628 0
>
> zsmalloc classes
>
> class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
> ...
> 151 2448 0 0 1240 1240 744 3 0
> 168 2720 0 0 4200 4200 2800 2 0
> 190 3072 0 0 10100 10100 7575 3 0
> 202 3264 0 0 7180 7180 5744 4 0
> 254 4096 0 0 3820 3820 3820 1 0
>
> Total 8 45 106959 106193 47424 0
>
> As we can see, we reduced the number of objects stored in class-4096,
> because a huge number of objects which we previously forcibly stored
> in class-4096 now stored in non-huge class-3264. This results in lower
> memory consumption:
> - zsmalloc now uses 47424 physical pages, which is less than 48787
> pages zsmalloc used before.
>
> - objects that we store in class-3264 share zspages. That's why overall
> the number of pages that both class-4096 and class-3264 consumed went
> down from 10924 to 9564.
>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
> drivers/block/zram/zram_drv.c | 9 ++++++++-
> drivers/block/zram/zram_drv.h | 16 ----------------
> 2 files changed, 8 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 85110e7931e5..1b8082e6d2f5 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -44,6 +44,11 @@ static const char *default_compressor = "lzo";
>
> /* Module params (documentation at end) */
> static unsigned int num_devices = 1;
> +/*
> + * Pages that compress to sizes equals or greater than this are stored
> + * uncompressed in memory.
> + */
> +static size_t huge_class_size;
>
> static void zram_free_page(struct zram *zram, size_t index);
>
> @@ -786,6 +791,8 @@ static bool zram_meta_alloc(struct zram *zram, u64 disksize)
> return false;
> }
>
> + if (!huge_class_size)
> + huge_class_size = zs_huge_class_size();

If it is static, we can do this in zram_init? I believe it's more readable in that
it's never changed betweens zram instances.

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