Does the Thundering Herd Problem exist on Linux anymore?

Question

Many linux/unix programming books and tutorials speak about the "Thundering Herd Problem" which happens when multiple threads or forks are blocked on a select() call waiting for readability of a listening socket. When the connection comes in, all threads and forks are woken up but only one "wins" with a successful call to "accept()". In the meantime, a lot of cpu time is wasted waking up all the threads/forks for no reason.

I noticed a project which provides a "fix" for this problem in the linux kernel, but this is a very old patch.

I think there are two variants; One where each fork does select() and then accept(), and one that just does accept().

Do modern unix/linux kernels still have the Thundering Herd Problem in both these cases or only the "select() then accept()" version?

Answer 1

For years, most unix/linux kernels serialize response to accept(2)s, in other words, only one thread is waken up if more than one are blocking on accept(2) against a single open file descriptor.

OTOH, many (if not all) kernels still have the thundering herd problem in the select-accept pattern as you describe.

I have written a simple script ( https://gist.github.com/kazuho/10436253 ) to verify the existence of the problem, and found out that the problem exists on linux 2.6.32 and Darwin 12.5.0 (OS X 10.8.5).