Re: epoll clarification sought: multithreaded epoll_wait for UDPsockets?

From: Andy Lutomirski
Date: Thu Mar 04 2010 - 08:20:01 EST

bert hubert wrote:
Dear kernel people, dear Davide,

I am currently debugging performance issues in the PowerDNS Recursor, and it
turns out I have been using epoll_wait() sub-optimally. And I need your help
to improve this. I'm more than happy to update the epoll_wait() manpage to
reflect your advice.

Essentially, what I would like to have is a way to distribute incoming UDP DNS
queries to various threads automatically. Right now, there is one
fd that multiple threads wait on, using epoll() or select() and subsequently
recvfrom(). Crucially, each thread has its own epoll fd set (which is

The hope is that each thread hogs a single CPU, and that UDP DNS queries
coming in arrive at a single thread that is currently in epoll_wait(), ie
not doing other things.

As indicated by the manpage of epoll however, my setup means that threads
get woken up unnecessarily when a new packet comes in. This results in lots
of recvfrom() calls returning EAGAIN (basically on most of the other

(this can be observed in )

The alternative appears to be to create a single epoll set, and have all
threads call epoll_wait on that same set.

The epoll() manpage however is silent on what this will do exactly, although
several LKML posts indicate that this might cause 'thundering herd'

My question is: what is your recommendation for achieving the scenario
outlined above? In other words, that is the 'best current practice' on
modern Linux kernels to get each packet to arrive at a single thread?

Epoll offers 'edge triggered' behaviour, would this make sense? Would it be
smart to cal epoll_wait with only a single event to be returned to prevent
starvation? Might it be useful to dup() the single fd, once for each thread? I also tried SO_REUSEADDR, so I could bind() multiple times to the same IP
address & port, but this does not distribute incoming queries.

EPOLLET sounds like the right approach. But if you don't want to drain the entire buffer in one thread and you use EPOLLET, you'll have to manually wake another thread. You could use eventfd for that, or maybe add a syscall to inject a new edge on a file descriptor. Good luck.

(FWIW, a couple days ago I got one machine handling over 9 GBps of incoming UDP packets on a single core without drops. It's a hefty machine, and I was using jumbo frames, but still.)

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