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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2012-04-23 at 09:42 -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> 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,
> but becomes a major source of real-time latency spikes on systems with
> many hundreds of CPUs.  In addition, the workloads running on these large
> systems tend to be CPU-bound, which eliminates the energy-efficiency
> advantages of synchronizing scheduling-clock interrupts.  Therefore,
> these systems need maximal values for the rcu_node leaf-level fanout.
> 
> This commit addresses this problem by introducing a new kernel parameter
> named RCU_FANOUT_LEAF that directly controls the leaf-level fanout.
> This parameter defaults to 16 to handle the common case of a moderate
> sized lightly loaded servers, but may be set higher on larger systems.

Wouldn't it be much better to match the rcu fanout tree to the physical
topology of the machine?