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

[Michal Hocko]

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
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