RCU (Read-Copy-Update)

RCU supports concurrency between a single updater and multiple readers. RCU ensures that reads are coherent by maintaining multiple versions of objects and ensuring that they are not freed up until all pre-existing read-side critical sections complete. RCU defines and uses efficient and scalable mechanisms for publishing and reading new versions of an object, and also for deferring the collection of old versions. These mechanisms distribute the work among read and update paths in such a way as to make read paths extremely fast.

Reader-writer locking and RCU simply provide different guarantees. With reader-writer locking, any reader that begins after the writer starts executing is guaranteed to see new values, and readers that attempt to start while the writer is spinning might or might not see new values, depending on the reader/writer preference of the rwlock implementation in question. In contrast, with RCU, any reader that begins after the updater completes is guaranteed to see new values, and readers that end after the updater begins might or might not see new values, depending on timing.

Advantages of RCU include performance, deadlock immunity, and realtime latency. There are, of course, limitations to RCU, including the fact that readers and updaters run concurrently, that low-priority RCU readers can block high-priority threads waiting for a grace period to elapse, and that grace-period latencies can extend for many milliseconds. These advantages and limitations are discussed in the following sections.

  1. Publish-Subscribe Mechanism (for insertion)
  2. Wait For Pre-Existing RCU Readers to Complete (for deletion)
  3. Maintain Multiple Versions of Recently Updated Objects (for readers)

Reference

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