On Wed 07-01-15 11:17:07, Mel Gorman wrote: > On Wed, Jan 07, 2015 at 11:57:49AM +0100, Michal Hocko wrote: > > 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. > > > > IIRC, the difference was a few percent on instruction profiles and cache > profiles when driven from a systemtap microbenchmark but I no longer have > the data and besides it would have been based on an ancient machine by > todays standards. When zeroing of pages is taken into account it's going > to be marginal so a userspace test would probably show nothing. Still, > I see little motivation to replace a single deference with multiple > dereferences and pointer arithmetic when zonelist_zone_idx() is called. OK, fair enough. I have tried to convert back to simple zone * and it turned out we wouldn't save too much code so this is really not worth time and possible complications. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>