Messages in this thread | | | Date | Thu, 26 Apr 2012 07:12:13 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 6/6] rcu: Reduce cache-miss initialization latencies for large systems |
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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
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