Summary:
RCU is a synchronization mechanism that allows for very fast reads, at the
expense of slower writes. This is achieved by having the reader sometimes
reading a stale pointer when concurrent to a write, at which point the writer
will delay reclaiming the old data to a later moment where it is known that no
reader can hold a pointer to the old data.
Doing that allows for the read operations to be significantly faster than using
a Synchronized lock. Folly's documentation claims a read lock/unlock of RCU
runs in ~4ns, while the same for Synchronized is ~26ns.
Due to the writers cost, RCU is perfectly suited for places where reads needs
to be as fast as possible, and writes are very infrequent. One typical example
is when caching an application's configuration, we can expect reading the
configuration values more frequently than it is being reloaded, and in the case
where the configuration mismatch, a stale configuration can be tolerated by the
application.
In EdenFS, we can use RCU on Windows to make sure that unmounting a repository
will wait on all the pending callbacks.
Reviewed By: kmancini
Differential Revision: D25351536
fbshipit-source-id: 050ca0337e67ae195f4f16062dddb60f584af692
