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

Robert Jennings
2011-01-26T18:50:43Z

* Pekka Enberg <penberg@kernel.org> wrote:
> 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
<snip>
>> @@ -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?

Yes, it does not need to be there. I will repost.