summaryrefslogtreecommitdiff
path: root/_posts/2013-04-09-erlang-tcp-socket-pull-pattern.md
diff options
context:
space:
mode:
Diffstat (limited to '_posts/2013-04-09-erlang-tcp-socket-pull-pattern.md')
-rw-r--r--_posts/2013-04-09-erlang-tcp-socket-pull-pattern.md256
1 files changed, 256 insertions, 0 deletions
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