Re: [PATCHv2 4/9] staging: zsmalloc: make CLASS_DELTA relative to PAGE_SIZE

[Seth Jennings <sjenning@linux.vnet.ibm.com>]

On 01/24/2013 06:17 PM, Nitin Gupta wrote:
> On Mon, Jan 7, 2013 at 12:24 PM, Seth Jennings
> <sjenning@linux.vnet.ibm.com> wrote:
>> Right now ZS_SIZE_CLASS_DELTA is hardcoded to be 16.  This
>> creates 254 classes for systems with 4k pages. However, on
>> PPC64 with 64k pages, it creates 4095 classes which is far
>> too many.
>>
>> This patch makes ZS_SIZE_CLASS_DELTA relative to PAGE_SIZE
>> so that regardless of the page size, there will be the same
>> number of classes.
>>
>> Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
>> ---
>>  drivers/staging/zsmalloc/zsmalloc-main.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
>> index 825e124..3543047 100644
>> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
>> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
>> @@ -141,7 +141,7 @@
>>   *  ZS_MIN_ALLOC_SIZE and ZS_SIZE_CLASS_DELTA must be multiple of ZS_ALIGN
>>   *  (reason above)
>>   */
>> -#define ZS_SIZE_CLASS_DELTA    16
>> +#define ZS_SIZE_CLASS_DELTA    (PAGE_SIZE >> 8)
>>  #define ZS_SIZE_CLASSES                ((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / \
>>                                         ZS_SIZE_CLASS_DELTA + 1)
>>
> 
> Actually, there is no point creating size classes beyond [M/(M+1)] * PAGE_SIZE
> where M is the maximum number of system pages in a zspage.

Agreed.

> All size classes
> beyond this size can be collapsed with PAGE_SIZE size class.  This can
> significantly reduce number of size classes created but I think changes needed
> to do this would be more involved, so perhaps, should be done in another
> patch.

I agree there could be some optimization here, but those extra classes
really aren't doing any harm that I can see.

> Can you please resend part of this series  (patch 1  to patch 4) which deals
> just with zsmalloc separately?  I haven't yet looked into zswap itself so would
> help with zsmalloc bits are separated out.

Working it now.

Thanks,
Seth

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