Sunday, May 26, 2013

Is it live, or is it structured deferral?

A delightful ACM paper by Paul E. McKenney, Structured Deferral: Synchronization via Procrastination. The "motivating example":

In this example, Schrödinger would like to construct an in-memory database to keep track of the animals in his zoo. Births would of course result in insertions into this database, while deaths would result in deletions. The database is also queried by those interested in the health and welfare of Schrödinger's animals.

Schrödinger has numerous short-lived animals such as mice, resulting in high update rates. In addition, there is a surprising level of interest in the health of Schrödinger's cat,19 so much so that Schrödinger sometimes wonders whether his mice are responsible for most of these queries. Regardless of their source, the database must handle the large volume of cat-related queries without suffering from excessive levels of contention. Both accesses and updates are typically quite short, involving accessing or mutating an in-memory data structure, and therefore synchronization overhead cannot be ignored.

Schrödinger also understands, however, that it is impossible to determine exactly when a given animal is born or dies. For example, suppose that his cat's passing is to be detected by heartbeat. Seconds or even minutes will be required to determine that the poor cat's heart has in fact stopped. The shorter the measurement interval, the less certain the measurement, so that a pair of veterinarians examining the cat might disagree on exactly when death occurred. For example, one might declare death 30 seconds after the last heartbeat, while another might insist on waiting a full minute, in which case the veterinarians disagree on the state of the cat during the second half of the minute after the last heartbeat.

Fortunately, Heisenberg8 has taught Schrödinger how to cope with such uncertainty. Furthermore, the delay in detecting the cat's passing permits use of synchronization via procrastination. After all, given that the two veterinarians' pronouncements of death were separated by a full 30 seconds, a few additional milliseconds of software procrastination is perfectly acceptable.

Among the pleasures in this paper: Linux's RCU, hazard pointers, and "read-only cat-heavy performance of Schrödinger's hash table".

Enjoy!

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