Re: [PATCH RFC tip/core/rcu 6/6] rcu: Reduce cache-missinitialization latencies for large systems

[Mike Galbraith]

On Mon, 2012-04-23 at 09:42 -0700, Paul E. McKenney wrote: 
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> Commit #0209f649 (rcu: limit rcu_node leaf-level fanout) set an upper
> limit of 16 on the leaf-level fanout for the rcu_node tree.  This was
> needed to reduce lock contention that was induced by the synchronization
> of scheduling-clock interrupts, which was in turn needed to improve
> energy efficiency for moderate-sized lightly loaded servers.
> 
> However, reducing the leaf-level fanout means that there are more
> leaf-level rcu_node structures in the tree, which in turn means that
> RCU's grace-period initialization incurs more cache misses.  This is
> not a problem on moderate-sized servers with only a few tens of CPUs,

With a distro config (4096 CPUs) interrupt latency is bad even on a
quad.  Traversing empty nodes taking locks and cache misses hurts.

-Mike

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