Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Read-copy-update is an operating system kernel technology for improving performance on computers with more than one CPU. More technically it is a synchronization mechanism which can sometimes be used as an alternative to a readers-writer lock. It allows extremely low overhead, wait-free reads. However, RCU updates can be expensive, as they must leave the old versions of the data structure in place to accommodate pre-existing readers. These old versions are reclaimed after all pre-existing readers finish their accesses. RCU can be put to a number of other tasks, including dynamically changing NMI handlers and implementing lazy barriers, but it is most frequently used as replacement for reader-writer locking.