Re: [RFC v1 00/19] Modify zonelist to nodelist v1

Qian Cai
2019-11-22T15:53:27Z

On Fri, 2019-11-22 at 23:28 +0800, Pengfei Li wrote:
> On Fri, 22 Nov 2019 15:25:00 +0800
> "lixinhai.lxh@gmail.com" <lixinhai.lxh@gmail.com> wrote:
>
> > On 2019-11-21 at 23:17 Pengfei Li wrote:
> > > Motivation
> > > ----------
> > > Currently if we want to iterate through all the nodes we have to
> > > traverse all the zones from the zonelist.
> > >
> > > So in order to reduce the number of loops required to traverse node,
> > > this series of patches modified the zonelist to nodelist.
> > >
> > > Two new macros have been introduced:
> > > 1) for_each_node_nlist
> > > 2) for_each_node_nlist_nodemask
> > >
> > >
> > > Benefit
> > > -------
> > > 1. For a NUMA system with N nodes, each node has M zones, the number
> > >    of loops is reduced from N*M times to N times when traversing
> > > node.
> > >
> >
> > It looks to me that we don't really have system which has N nodes and
> > each node with M zones in its address range.
> > We may have systems which has several nodes, but only the first node
> > has all zone types, other nodes only have NORMAL zone. (Evenly
> > distribute the !NORMAL zones on all nodes is not reasonable, as those
> > zones have limited size)
> > So iterate over zones to reach nodes should at N level, not M*N level.
> >
>
> Thanks for your comments.
>
> In the case you said, the number of loops required to traverse all
> nodes is similar to traversing all zones.
>
> I have two main reasons to explain that this series of patches is
> beneficial.
>
> 1. When node has more than one zone, it will take fewer cycles to
> traverse all nodes. (for example, ZONE_MOVABLE?)

ZONE_MOVABLE is broken for ages (non-movable allocations are there all the time
last time I tried) which indicates there is very few people care about it, so it
is rather weak to use that as a justification for the churns it might cause.

>
> 2. Using zonelist to traverse all nodes is inefficient, pgdat must be
> obtained indirectly via zone->zone_pgdat, and additional judgment must
> be made.
>
> E.g
> 1) Using zonelist to traverse all nodes
>
> last_pgdat = NULL;
> for_each_zone_zonelist(zone, xxx) {
> pgdat = zone->zone_pgdat;
> if (pgdat == last_pgdat)
> continue;
>
> last_pgdat = pgdat;
> do_something(pgdat);
> }
>
> 2) Using nodelist to traverse all nodes
>
> for_each_node_nodelist(node, xxx) {
> do_something(NODE_INFO(node));
> }
>