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

[Nitin Gupta]

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@xxxxxxxxxx>
> > > > ---
> > > >    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
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