Re: [PATCH V4 4/4] mm: microoptimize zonelist operations

[Michal Hocko <mhocko@suse.cz>]

On Wed 07-01-15 10:15:39, Vlastimil Babka wrote:
> On 01/06/2015 04:09 PM, Michal Hocko wrote:
> > On Mon 05-01-15 18:17:43, Vlastimil Babka wrote:
> >> The function next_zones_zonelist() returns zoneref pointer, as well as zone
> >> pointer via extra parameter. Since the latter can be trivially obtained by
> >> dereferencing the former, the overhead of the extra parameter is unjustified.
> >> 
> >> This patch thus removes the zone parameter from next_zones_zonelist(). Both
> >> callers happen to be in the same header file, so it's simple to add the
> >> zoneref dereference inline. We save some bytes of code size.
> > 
> > Dunno. It makes first_zones_zonelist and next_zones_zonelist look
> > different which might be a bit confusing. It's not a big deal but
> > I am not sure it is worth it.
> 
> Yeah I thought that nobody uses them directly anyway thanks to
> for_each_zone_zonelist* so it's not a big deal.

OK, I have checked why we need the whole struct zoneref when it
only caches zone_idx. dd1a239f6f2d (mm: have zonelist contains
structs with both a zone pointer and zone_idx) claims this will
reduce cache contention by reducing pointer chasing because we
do not have to dereference pgdat so often in hot paths. Fair
enough but I do not see any numbers in the changelog nor in the
original discussion (https://lkml.org/lkml/2007/11/20/547 resp.
https://lkml.org/lkml/2007/9/28/170). Maybe Mel remembers what was the
benchmark which has shown the difference so that we can check whether
this is still relevant and caching the index is still worth it.

-- 
Michal Hocko
SUSE Labs