Messages in this thread | | | Date | Wed, 19 Apr 2017 07:50:53 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick |
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On Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote: > On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote: > > > I believe that you are missing the fact that RCU grace-period > > initialization and cleanup walks through the rcu_node tree breadth > > first, using rcu_for_each_node_breadth_first(). > > Indeed. That is the part I completely missed. > > > This macro (shown below) > > implements this breadth-first walk using a simple sequential traversal of > > the ->node[] array that provides the structures making up the rcu_node > > tree. As you can see, this scan is completely independent of how CPU > > numbers might be mapped to rcu_data slots in the leaf rcu_node structures. > > So this code is clearly not a hotpath, but still its performance > matters? > > Seems like you cannot win here :/
Welcome to my world!!! ;-)
But yes, running on 4096-CPU systems can put some serious stress on some surprising areas. Especially when those systems have cache-miss latencies well in excess of a microsecond, and the users are nevertheless expecting scheduling latencies well below 100 microseconds.
It was a fun challenge, I grant you that!
Thanx, Paul
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