Re: [PATCH 1/7] zram/vmalloc: Correct tunings to enable use with 64K pages
From: Pekka Enberg
Date: Wed Jan 26 2011 - 12:48:49 EST
On Wed, Jan 26, 2011 at 7:22 PM, Robert Jennings <rcj@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> ---
> 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
--
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