10.3.2.3 RCU is a Bulk Reference-Counting Mechanism
Keywords: perfbook_html

###
10.3.2.3 RCU is a Bulk Reference-Counting Mechanism

As noted in the preceding section, traditional reference counters are usually associated with a specific data structure, or perhaps a specific group of data structures. However, maintaining a single global reference counter for a large variety of data structures typically results in bouncing the cache line containing the reference count. Such cache-line bouncing can severely degrade performance.

In contrast, RCU's light-weight read-side primitives permit extremely frequent read-side usage with negligible performance degradation, permitting RCU to be used as a "bulk reference-counting" mechanism with little or no performance penalty. Situations where a reference must be held by a single task across a section of code that blocks may be accommodated with Sleepable RCU (SRCU) [McK06]. This fails to cover the not-uncommon situation where a reference is "passed" from one task to another, for example, when a reference is acquired when starting an I/O and released in the corresponding completion interrupt handler. (In principle, this could be handled by the SRCU implementation, but in practice, it is not yet clear whether this is a good tradeoff.)

Of course, SRCU brings restrictions of its own, namely that the return value from srcu_read_lock() be passed into the corresponding srcu_read_unlock(), and that no SRCU primitives be invoked from hardware irq handlers or from NMI/SMI handlers. The jury is still out as to how much of a problem is presented by these restrictions, and as to how they can best be handled.