Exploiting deferred destruction: an analysis of read-copy-update techniques in operating system kernels

• Authors: Paul E. Mckenney;Jonathan Walpole
• Venue: Exploiting deferred destruction: an analysis of read-copy-update techniques in operating system kernels
• Year: 2004

The Moore's-Law-driven performance of simple instructions has improved by orders of magnitude over the past two decades, but shared-memory multiprocessor (SMMP) synchronization operations have not kept pace. SMMP software uses synchronization operations heavily, thus suffering degraded performance and scalability. As a result, many traditional SMMP algorithms are now obsolete. This dissertation presents read-copy update (RCU), a reader-writer synchronization mechanism in which read-side critical sections incur virtually zero synchronization overhead, thereby achieving near-ideal performance for read-mostly workloads. Write-side critical sections incur substantial synchronization overhead, deferring destruction and maintaining multiple versions of data structures in order to accommodate the synchronization-free read-side critical sections. In addition, writers use some mechanism, such as locking, to ensure orderly updates. Readers provide a signal enabling writers to determine when it is safe to complete destructive operations, but this signal may be deferred, permitting a single signal operation to serve multiple read-side RCU critical sections. These read-side signals are observed by a specialized garbage collector, which carries out destructive operations once it is safe to do so. Garbage collectors are typically implemented in a manner similar to a barrier computation. Production-quality garbage collectors batch destructive operations, amortizing signal-observation overhead over many updates. Although RCU is not itself new, its use has been quite specialized. This dissertation rectifies this situation by showing how RCU can be implemented efficiently in operating system kernels, by demonstrating its system-level performance and complexity benefits, and by providing a set of design patterns that make RCU more generally applicable. This dissertation compares RCU to traditional synchronization mechanisms, including locking and non-blocking synchronization, using both analytic and empirical methods. The empirical methods include both informal micro-benchmarks and formal system-level benchmarks. These benchmarks show performance benefits ranging from tens of percent to an order of magnitude and little or no increase in code complexity. Finally, this dissertation demonstrates that RCU has practical value by (1) outlining its use in several production systems, two of which have seen extensive datacenter use, one of which this author designed and implemented, and (2) documenting its widespread use in the Linux 2.6 kernel.