Re: [PATCH 4/4] zsmalloc: zsmalloc: align cache line size

[Nitin Gupta <ngupta@vflare.org>]

On 5/7/12 3:41 AM, Pekka Enberg wrote:
> On Fri, 4 May 2012, Minchan Kim wrote:
>>>> It's a overkill to align pool size with PAGE_SIZE to avoid
>>>> false-sharing. This patch aligns it with just cache line size.
>>>>
>>>> Signed-off-by: Minchan Kim<minchan@kernel.org>
>>>> ---
>>>>    drivers/staging/zsmalloc/zsmalloc-main.c |    6 +++---
>>>>    1 file changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c
>>>> b/drivers/staging/zsmalloc/zsmalloc-main.c
>>>> index 51074fa..3991b03 100644
>>>> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
>>>> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
>>>> @@ -489,14 +489,14 @@ fail:
>>>>
>>>>    struct zs_pool *zs_create_pool(const char *name, gfp_t flags)
>>>>    {
>>>> -    int i, error, ovhd_size;
>>>> +    int i, error;
>>>>        struct zs_pool *pool;
>>>>
>>>>        if (!name)
>>>>            return NULL;
>>>>
>>>> -    ovhd_size = roundup(sizeof(*pool), PAGE_SIZE);
>>>> -    pool = kzalloc(ovhd_size, GFP_KERNEL);
>>>> +    pool = kzalloc(ALIGN(sizeof(*pool), cache_line_size()),
>>>> +                GFP_KERNEL);
>>>
>>> a basic question:
>>>   Is rounding off allocation size to cache_line_size enough to ensure
>>> that the object is cache-line-aligned? Isn't it possible that even
>>> though the object size is multiple of cache-line, it may still not be
>>> properly aligned and end up sharing cache line with some other
>>> read-mostly object?
>>
>> AFAIK, SLAB allocates object aligned cache-size so I think that problem cannot happen.
>> But needs double check.
>> Cced Pekka.
>
> The kmalloc(size) function only gives you the following guarantees:
>
>    (1) The allocated object is _at least_ 'size' bytes.
>
>    (2) The returned pointer is aligned to ARCH_KMALLOC_MINALIGN.
>
> Anything beyond that is implementation detail and probably will break if
> you switch between SLAB/SLUB/SLOB.
>
> 			Pekka

So, we can probably leave it as is (PAGE_SIZE aligned) or use 
kmem_cache_create(...,SLAB_HWCACHE_ALIGN,...) for allocating 'struct 
zs_pool's.

zcache can potentially create a lot of pools, so the latter will save 
some memory.

Thanks,
Nitin