[$] A ring buffer for epoll

The set of system calls known collectively as epoll was
designed to make polling for I/O events more scalable. To that end, it
minimizes the amount of setup that must be done for each system call and
returns multiple events so that the number of calls can also be minimized.
But that turns out to still not be scalable enough for some users. The
response to this problem, in the form of this patch
series from Roman Penyaev, takes a familiar form: add yet another
ring-buffer interface to the kernel.