From: Paul E. McKenney Date: Thu, 19 Apr 2012 19:20:14 +0000 (-0700) Subject: rcu: Reduce cache-miss initialization latencies for large systems X-Git-Tag: v3.5-rc1~164^2^2^2~3 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8932a63d5edb02f714d50c26583152fe0a97a69c;p=pandora-kernel.git rcu: Reduce cache-miss initialization latencies for large systems 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. Reported-by: Mike Galbraith Reported-by: Dimitri Sivanich Signed-off-by: Paul E. McKenney --- Reading git-diff-tree failed