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diff --git a/_posts/2013-04-09-erlang-tcp-socket-pull-pattern.md b/_posts/2013-04-09-erlang-tcp-socket-pull-pattern.md new file mode 100644 index 0000000..3e5f0af --- /dev/null +++ b/_posts/2013-04-09-erlang-tcp-socket-pull-pattern.md @@ -0,0 +1,256 @@ +--- +title: "Erlang, tcp sockets, and active true" +description: >- + Using `{active:once}` isn't always the best way to handle connections. +--- + +If you don't know erlang then [you're missing out][0]. If you do know erlang, +you've probably at some point done something with tcp sockets. Erlang's highly +concurrent model of execution lends itself well to server programs where a high +number of active connections is desired. Each thread can autonomously handle its +single client, greatly simplifying the logic of the whole application while +still retaining [great performance characteristics][1]. + +## Background + +For an erlang thread which owns a single socket there are three different ways +to receive data off of that socket. These all revolve around the `active` +[setopts][2] flag. A socket can be set to one of: + +* `{active,false}` - All data must be obtained through [recv/2][3] calls. This + amounts to syncronous socket reading. + +* `{active,true}` - All data on the socket gets sent to the controlling thread + as a normal erlang message. It is the thread's + responsibility to keep up with the buffered data in the + message queue. This amounts to asyncronous socket reading. + +* `{active,once}` - When set the socket is placed in `{active,true}` for a + single packet. That is, once set the thread can expect a + single message to be sent to when data comes in. To receive + any more data off of the socket the socket must either be + read from using [recv/2][3] or be put in `{active,once}` or + `{active,true}`. + +## Which to use? + +Many (most?) tutorials advocate using `{active,once}` in your application +\[0]\[1]\[2]. This has to do with usability and security. When in `{active,true}` +it's possible for a client to flood the connection faster than the receiving +process will process those messages, potentially eating up a lot of memory in +the VM. However, if you want to be able to receive both tcp data messages as +well as other messages from other erlang processes at the same time you can't +use `{active,false}`. So `{active,once}` is generally preferred because it +deals with both of these problems quite well. + +## Why not to use `{active,once}` + +Here's what your classic `{active,once}` enabled tcp socket implementation will +probably look like: + +```erlang +-module(tcp_test). +-compile(export_all). + +-define(TCP_OPTS, [ + binary, + {packet, raw}, + {nodelay,true}, + {active, false}, + {reuseaddr, true}, + {keepalive,true}, + {backlog,500} +]). + +%Start listening +listen(Port) -> + {ok, L} = gen_tcp:listen(Port, ?TCP_OPTS), + ?MODULE:accept(L). + +%Accept a connection +accept(L) -> + {ok, Socket} = gen_tcp:accept(L), + ?MODULE:read_loop(Socket), + io:fwrite("Done reading, connection was closed\n"), + ?MODULE:accept(L). + +%Read everything it sends us +read_loop(Socket) -> + inet:setopts(Socket, [{active, once}]), + receive + {tcp, _, _} -> + do_stuff_here, + ?MODULE:read_loop(Socket); + {tcp_closed, _}-> donezo; + {tcp_error, _, _} -> donezo + end. +``` + +This code isn't actually usable for a production system; it doesn't even spawn a +new process for the new socket. But that's not the point I'm making. If I run it +with `tcp_test:listen(8000)`, and in other window do: + +```bash +while [ 1 ]; do echo "aloha"; done | nc localhost 8000 +``` + +We'll be flooding the the server with data pretty well. Using [eprof][4] we can +get an idea of how our code performs, and where the hang-ups are: + +```erlang +1> eprof:start(). +{ok,<0.34.0>} + +2> P = spawn(tcp_test,listen,[8000]). +<0.36.0> + +3> eprof:start_profiling([P]). +profiling + +4> running_the_while_loop. +running_the_while_loop + +5> eprof:stop_profiling(). +profiling_stopped + +6> eprof:analyze(procs,[{sort,time}]). + +****** Process <0.36.0> -- 100.00 % of profiled time *** +FUNCTION CALLS % TIME [uS / CALLS] +-------- ----- --- ---- [----------] +prim_inet:type_value_2/2 6 0.00 0 [ 0.00] + +....snip.... + +prim_inet:enc_opts/2 6 0.00 8 [ 1.33] +prim_inet:setopts/2 12303599 1.85 1466319 [ 0.12] +tcp_test:read_loop/1 12303598 2.22 1761775 [ 0.14] +prim_inet:encode_opt_val/1 12303599 3.50 2769285 [ 0.23] +prim_inet:ctl_cmd/3 12303600 4.29 3399333 [ 0.28] +prim_inet:enc_opt_val/2 24607203 5.28 4184818 [ 0.17] +inet:setopts/2 12303598 5.72 4533863 [ 0.37] +erlang:port_control/3 12303600 77.13 61085040 [ 4.96] +``` + +eprof shows us where our process is spending the majority of its time. The `%` +column indicates percentage of time the process spent during profiling inside +any function. We can pretty clearly see that the vast majority of time was spent +inside `erlang:port_control/3`, the BIF that `inet:setopts/2` uses to switch the +socket to `{active,once}` mode. Amongst the calls which were called on every +loop, it takes up by far the most amount of time. In addition all of those other +calls are also related to `inet:setopts/2`. + +I'm gonna rewrite our little listen server to use `{active,true}`, and we'll do +it all again: + +```erlang +-module(tcp_test). +-compile(export_all). + +-define(TCP_OPTS, [ + binary, + {packet, raw}, + {nodelay,true}, + {active, false}, + {reuseaddr, true}, + {keepalive,true}, + {backlog,500} +]). + +%Start listening +listen(Port) -> + {ok, L} = gen_tcp:listen(Port, ?TCP_OPTS), + ?MODULE:accept(L). + +%Accept a connection +accept(L) -> + {ok, Socket} = gen_tcp:accept(L), + inet:setopts(Socket, [{active, true}]), %Well this is new + ?MODULE:read_loop(Socket), + io:fwrite("Done reading, connection was closed\n"), + ?MODULE:accept(L). + +%Read everything it sends us +read_loop(Socket) -> + %inet:setopts(Socket, [{active, once}]), + receive + {tcp, _, _} -> + do_stuff_here, + ?MODULE:read_loop(Socket); + {tcp_closed, _}-> donezo; + {tcp_error, _, _} -> donezo + end. +``` + +And the profiling results: + +```erlang +1> eprof:start(). +{ok,<0.34.0>} + +2> P = spawn(tcp_test,listen,[8000]). +<0.36.0> + +3> eprof:start_profiling([P]). +profiling + +4> running_the_while_loop. +running_the_while_loop + +5> eprof:stop_profiling(). +profiling_stopped + +6> eprof:analyze(procs,[{sort,time}]). + +****** Process <0.36.0> -- 100.00 % of profiled time *** +FUNCTION CALLS % TIME [uS / CALLS] +-------- ----- --- ---- [----------] +prim_inet:enc_value_1/3 7 0.00 1 [ 0.14] +prim_inet:decode_opt_val/1 1 0.00 1 [ 1.00] +inet:setopts/2 1 0.00 2 [ 2.00] +prim_inet:setopts/2 2 0.00 2 [ 1.00] +prim_inet:enum_name/2 1 0.00 2 [ 2.00] +erlang:port_set_data/2 1 0.00 2 [ 2.00] +inet_db:register_socket/2 1 0.00 3 [ 3.00] +prim_inet:type_value_1/3 7 0.00 3 [ 0.43] + +.... snip .... + +prim_inet:type_opt_1/1 19 0.00 7 [ 0.37] +prim_inet:enc_value/3 7 0.00 7 [ 1.00] +prim_inet:enum_val/2 6 0.00 7 [ 1.17] +prim_inet:dec_opt_val/1 7 0.00 7 [ 1.00] +prim_inet:dec_value/2 6 0.00 10 [ 1.67] +prim_inet:enc_opt/1 13 0.00 12 [ 0.92] +prim_inet:type_opt/2 19 0.00 33 [ 1.74] +erlang:port_control/3 3 0.00 59 [ 19.67] +tcp_test:read_loop/1 20716370 100.00 12187488 [ 0.59] +``` + +This time our process spent almost no time at all (according to eprof, 0%) +fiddling with the socket opts. Instead it spent all of its time in the +read_loop doing the work we actually want to be doing. + +## So what does this mean? + +I'm by no means advocating never using `{active,once}`. The security concern is +still a completely valid concern and one that `{active,once}` mitigates quite +well. I'm simply pointing out that this mitigation has some fairly serious +performance implications which have the potential to bite you if you're not +careful, especially in cases where a socket is going to be receiving a large +amount of traffic. + +## Meta + +These tests were done using R15B03, but I've done similar ones in R14 and found +similar results. I have not tested R16. + +* \[0] http://learnyousomeerlang.com/buckets-of-sockets +* \[1] http://www.erlang.org/doc/man/gen_tcp.html#examples +* \[2] http://erlycoder.com/25/erlang-tcp-server-tcp-client-sockets-with-gen_tcp + +[0]: http://learnyousomeerlang.com/content +[1]: http://www.metabrew.com/article/a-million-user-comet-application-with-mochiweb-part-1 +[2]: http://www.erlang.org/doc/man/inet.html#setopts-2 +[3]: http://www.erlang.org/doc/man/gen_tcp.html#recv-2 +[4]: http://www.erlang.org/doc/man/eprof.html |