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| -rw-r--r-- | erlang-tcp-socket-pull-pattern.md | 184 |
1 files changed, 182 insertions, 2 deletions
diff --git a/erlang-tcp-socket-pull-pattern.md b/erlang-tcp-socket-pull-pattern.md index c17a799..d8a60d9 100644 --- a/erlang-tcp-socket-pull-pattern.md +++ b/erlang-tcp-socket-pull-pattern.md @@ -26,8 +26,188 @@ flag. A socket can be set to one of: # Which to use? -<Explanation of how other sources claim you should use active,once, and why> +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 it +# 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](http://www.erlang.org/doc/man/eprof.html) 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 |