Re: [PATCH 2/6] zsmalloc: remove unnecessary alignment

[Nitin Gupta]

Hi Minchan,

Sorry for late reply.

On 4/25/12 9:42 PM, Minchan Kim wrote:
> On 04/25/2012 09:53 PM, Nitin Gupta wrote:
>
> > On 04/25/2012 02:23 AM, Minchan Kim wrote:
> >
> > > It isn't necessary to align pool size with PAGE_SIZE.
> > > If I missed something, please let me know it.
> > >
> > > Signed-off-by: Minchan Kim<minchan@xxxxxxxxxx>
> > > ---
> > >   drivers/staging/zsmalloc/zsmalloc-main.c |    5 ++---
> > >   1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
> > > index 504b6c2..b99ad9e 100644
> > > --- a/drivers/staging/zsmalloc/zsmalloc-main.c
> > > +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
> > > @@ -489,14 +489,13 @@ 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(sizeof(*pool), GFP_KERNEL);
> > >   	if (!pool)
> > >   		return NULL;
> >
> >
> > pool metadata is rounded-up to avoid potential false-sharing problem
> > (though we could just roundup to cache_line_size()).
>
>
> Do you really have any hurt by false-sharing problem?
> If so, we can change it with

I've never been hit by this false-sharing in any testing but this is 
really just a random chance. Apart from aligning to cache-line size, 
there is no way to ensure some unfortunate read-mostly object never 
falls in the same line.

> kzalloc(ALIGN(sizeof(*pool), cache_line_size()), GFP_KERNEL);

Yes, looks better than aligning to PAGE_SIZE.


Thanks,
Nitin
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