Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

[Peter Zijlstra]

On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote:
> On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote:
> > > > > To avoid people tuning huge machines having to wait for me to give
> > > > > them an answer as to why they are suffering lock contention after
> > > > > cranking up the value of RCU_FANOUT_LEAF.
> > 
> > So is there a good reason to increase FANOUT_LEAF ?
> 
> Increasing it reduces the number of rcu_node structures, and thus the
> number of cache misses during grace-period initialization and cleanup.
> This has proven necessary in the past on large machines having long
> memory latencies.  And there are starting to be some pretty big machines
> running in production, and even for typical commerical workloads.

Is that perhaps a good moment to look at aligning the cpus in said nodes
with the cache topology?