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fleshed out the rest of the erlang active once post

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mediocregopher 11 years ago
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      erlang-tcp-socket-pull-pattern.md

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erlang-tcp-socket-pull-pattern.md
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@ -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

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