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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Apr 26, 2012 at 02:51:47PM +0200, Peter Zijlstra wrote:
> 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?

>From what I am hearing, doing so requires me to morph the rcu_node tree
at run time.  I might eventually become courageous/inspired/senile
enough to try this, but not yet.  ;-)

Actually, some of this topology shifting seems to me like a firmware
bug.  Why not arrange the Linux-visible numbering in a way to promote
locality for code sequencing through the CPUs?

							Thanx, Paul