D.2 Hierarchical RCU Overview
Keywords: perfbook_html

#
D.2 Hierarchical RCU Overview

Although Classic RCU's read-side primitives enjoy excellent performance and scalability, the update-side primitives, which determine when pre-existing read-side critical sections have finished, were designed with only a few tens of CPUs in mind. Their scalability is limited by a global lock that must be acquired by each CPU at least once during each grace period. Although Classic RCU actually scales to a couple of hundred CPUs, and can be tweaked to scale to roughly a thousand CPUs (but at the expense of extending grace periods), emerging multicore systems will require it to scale better.

In addition, Classic RCU has a sub-optimal dynticks interface, with the result that Classic RCU will wake up every CPU at least once per grace period. To see the problem with this, consider a 16-CPU system that is sufficiently lightly loaded that it is keeping only four CPUs busy. In a perfect world, the remaining twelve CPUs could be put into deep sleep mode in order to conserve energy. Unfortunately, if the four busy CPUs are frequently performing RCU updates, those twelve idle CPUs will be awakened frequently, wasting significant energy. Thus, any major change to Classic RCU should also leave sleeping CPUs lie.

Both the classic and the hierarchical implementations have have Classic RCU semantics and identical APIs, however, the old implementation will be called ``classic RCU'' and the new implementation will be called ``hierarchical RCU''.

@@@ roadmap @@@

Subsections

- D.2.1 Review of RCU Fundamentals
- D.2.2 Brief Overview of Classic RCU Implementation
- D.2.3 RCU Desiderata
- D.2.4 Towards a More Scalable RCU Implementation
- D.2.5 Towards a Greener RCU Implementation
- D.2.6 State Machine
- D.2.7 Use Cases
  - D.2.7.1 Start a New Grace Period
  - D.2.7.2 Pass Through a Quiescent State
  - D.2.7.3 Announce a Quiescent State to RCU
  - D.2.7.4 Enter and Leave Dynticks Idle Mode
  - D.2.7.5 Interrupt from Dynticks Idle Mode
  - D.2.7.6 NMI from Dynticks Idle Mode
  - D.2.7.7 Note That a CPU is in Dynticks Idle Mode
  - D.2.7.8 Offline a CPU
  - D.2.7.9 Online a CPU
  - D.2.7.10 Detect a Too-Long Grace Period
- D.2.8 Testing
- D.2.9 Conclusion