Re: [PATCH 1/7] zram/vmalloc: Correct tunings to enable use with 64K pages

Pekka Enberg
2011-01-26T17:48:47Z

On Wed, Jan 26, 2011 at 7:22 PM, Robert Jennings <rcj@linux.vnet.ibm.com> wrote:
> xvmalloc will not currently function with 64K pages.  Newly allocated
> pages will be inserted at an offset beyond the end of the first-level
> index.  This tuning is needed to properly size the allocator for 64K
> pages.
>
> The default 3 byte shift results in a second level list size which can not
> be indexed using the 64 bits of the flbitmap in the xv_pool structure.
> The shift must increase to 4 bytes between second level list entries to
> fit the size of the first level bitmap.
>
> Here are a few statistics for structure sizes on 32- and 64-bit CPUs
> with 4KB and 64KB page sizes.
>
> bits_per_long              32        64        64
> page_size               4,096     4,096    65,535
> xv_align                    4         8         8
> fl_delta                    3         3         4
> num_free_lists            508       508     4,094
> xv_pool size            4,144b    8,216b   66,040b
> per object overhead        32        64        64
> zram struct 0.5GB disk    512KB    1024KB      64KB
>
> This patch maintains the current tunings for 4K pages, adds an optimal
> sizing for 64K pages and adds a safe tuning for any other page sizes.
>
> Signed-off-by: Robert Jennings <rcj@linux.vnet.ibm.com>
> ---
>  drivers/staging/zram/xvmalloc_int.h |   18 ++++++++++++++++--
>  1 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/staging/zram/xvmalloc_int.h b/drivers/staging/zram/xvmalloc_int.h
> index e23ed5c..051a49b 100644
> --- a/drivers/staging/zram/xvmalloc_int.h
> +++ b/drivers/staging/zram/xvmalloc_int.h
> @@ -19,7 +19,11 @@
>  /* User configurable params */
>
>  /* Must be power of two */
> +#ifdef CONFIG_64BIT
> +#define XV_ALIGN_SHIFT 3
> +#else
>  #define XV_ALIGN_SHIFT 2
> +#endif
>  #define XV_ALIGN       (1 << XV_ALIGN_SHIFT)
>  #define XV_ALIGN_MASK  (XV_ALIGN - 1)
>
> @@ -27,8 +31,18 @@
>  #define XV_MIN_ALLOC_SIZE      32
>  #define XV_MAX_ALLOC_SIZE      (PAGE_SIZE - XV_ALIGN)
>
> -/* Free lists are separated by FL_DELTA bytes */
> -#define FL_DELTA_SHIFT 3
> +/*
> + * Free lists are separated by FL_DELTA bytes
> + * This value is 3 for 4k pages and 4 for 64k pages, for any
> + * other page size, a conservative (PAGE_SHIFT - 9) is used.
> + */
> +#if PAGE_SHIFT == 12
> +#define FL_DELTA_SHIFT 3

This is handled by the else branch already, no?

> +#elif PAGE_SHIFT == 16
> +#define FL_DELTA_SHIFT 4
> +#else
> +#define FL_DELTA_SHIFT (PAGE_SHIFT - 9)
> +#endif