On 06/06/2013 07:21 AM, Dave Chinner wrote:
>> It's unclear that active_nodes is really needed - we could just iterate
>> > across all items in list_lru.node[].  Are we sure that the correct
>> > tradeoff decision was made here?
> Yup. Think of all the cache line misses that checking
> node[x].nr_items != 0 entails. If MAX_NUMNODES = 1024, there's 1024
> cacheline misses right there. The nodemask is a much more cache
> friendly method of storing active node state.
> 
> not to mention that for small machines with a large MAX_NUMNODES,
> we'd be checking nodes that never have items stored on them...
> 
>> > What's the story on NUMA node hotplug, btw?
> Do we care? hotplug doesn't change MAX_NUMNODES, and if you are
> removing a node you have to free all the memory on the node,
> so that should already be tken care of by external code....
> 

Mel have already complained about this.
I have a patch that makes it dynamic but I didn't include it in here
because the series was already too big. I was also hoping to get it
ontop of the others, to avoid disruption.

I am attaching here for your appreciation.

For the record, nr_node_ids is firmware provided and it is actually
possible nodes, not online nodes. So hotplug won't change that.