Socket Timeouts in Ruby
One of Ruby’s weaknesses is its poor networking performance. Much of that has to do with the net/http implementation, which uses Ruby’s awful Timeout library. The issues with Timeout are well documented. SystemTimer provides a reliable alternative that also performs better.
However I started today wondering if there was a better way. Enabling timeouts has a huge performance hit on my memcache-client library and reducing the overhead would go a long way to making it perform safely and quickly. Since C programs need socket timeouts also, I figured there had to be a low-level alternative, and indeed there is: the
SO_RCVTIMEO socket options. It’s a bit involved to create a proper socket with these options but possible:
def connect_to(host, port, timeout=nil) addr = Socket.getaddrinfo(host, nil) sock = Socket.new(Socket.const_get(addr), Socket::SOCK_STREAM, 0) if timeout secs = Integer(timeout) usecs = Integer((timeout - secs) * 1_000_000) optval = [secs, usecs].pack("l_2") sock.setsockopt Socket::SOL_SOCKET, Socket::SO_RCVTIMEO, optval sock.setsockopt Socket::SOL_SOCKET, Socket::SO_SNDTIMEO, optval end sock.connect(Socket.pack_sockaddr_in(port, addr)) sock end
There are a few complexities in the code:
- We use the low-level operations,
connectrather than just
TCPSocket.new(host, port)because otherwise we can’t set the socket options before the connection is attempted; we want to ensure the connection attempt itself is timed out also.
- We have to look up the host via DNS by hand as some systems (*cough*, OSX) can return either IPv6 or IPv4 addresses and the address family constant used in Socket.new must match the address used in the connect statement.
setsockoptmethod takes a native C struct so we need to construct it using the
Here’s the results, from worst to best:
== memcache-client 1.7.0 with Ruby 1.8.6, normal Ruby timeouts user system total real mixed:ruby:memcache-client 14.240000 7.470000 21.710000 ( 22.173267)
== memcache-client 1.7.0 with Ruby 1.8.6, SystemTimer 1.1.1 user system total real mixed:ruby:memcache-client 12.400000 1.960000 14.360000 ( 14.857924)
== memcache-client 1.7.0 with Ruby 1.8.6, raw socket timeouts user system total real mixed:ruby:memcache-client 2.750000 0.620000 3.370000 ( 5.841545)
== memcache-client 1.7.0 with Ruby 1.8.6, no socket timeouts user system total real mixed:ruby:memcache-client 2.760000 0.620000 3.380000 ( 5.902549)
Awesome. With raw socket timeouts, there is no performance impact! SystemTimer provides an excellent replacement for Timeout if you want to guarantee a ceiling on the time spent in an arbitrary block, but if you just need timeouts for low-level socket operations, nothing beats the operating system’s native socket timeout support.
There is a caveat in the paragraph above: low-level socket operations. memcache-client uses three IO methods: read, write and gets. The first two are low-level and time out properly, but gets is built on the low-level read operation; it has to ignore the EAGAIN error in order to ensure it returns a full line of text. So we use a hybrid approach, read and write will use the raw socket timeouts and gets will use SystemTimer. It’s not quite as fast as with no/raw timeouts but it’s definitely an improvement:
== memcache-client 1.7.0 with Ruby 1.8.6, raw socket timeouts and SystemTimer user system total real mixed:ruby:memcache-client 7.490000 1.270000 8.760000 ( 9.361547)
So we’ve gone from 22 sec with Timeout to 15 sec with SystemTimer to 9 sec using raw socket timeouts where possible (Github commit). For my next trick, I figure I’ll rewrite
gets to use read so I can remove the need for SystemTimer and Timeout altogether.