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