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

[Robert Jennings <rcj@linux.vnet.ibm.com>]

* 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.