[erlang-questions] gen_tcp very slow to fetch data

Ngoc Dao <>
Tue Nov 17 01:51:23 CET 2009


>From inet's doc:
http://www1.erlang.org/documentation/doc-4.9.1/lib/kernel-2.4.1/doc/html/inet.html

If the active option is true, which is the default, everything
received from the socket will be sent as messages to the receiving
process. If the active option is set to false (passive mode), the
process must explicitly receive incoming data by calling
gen_tcp:recv/N or gen_udp:recv/N  (depending on the type of socket).
Note: Passive mode provides flow control; the other side will not be
able send faster than the receiver can read. Active mode provides no
flow control; a fast sender could easily overflow the receiver with
incoming messages. Use active mode only if your high-level protocol
provides its own flow control (for instance, acknowledging received
messages) or the amount of data exchanged is small.


On Tue, Nov 17, 2009 at 2:59 AM, ERLANG <> wrote:
> Hi Chandru !
>
> That's fix my problem. Thanks.
> While googling a bit, I found two ways to read from the Socket:
>
> recv(Socket, Bin) ->
>    receive
>        {tcp, Socket, B} ->
>            io:format(".", []),
>            recv(Socket, concat_binary([Bin, B]));
>        {tcp_closed, Socket} ->
>            {ok, Bin};
>        Other ->
>            {error, {socket, Other}}
>        after
>            ?TIMEOUT ->
>            {error, {socket, timeout}}
>    end.
>
> % version 2 with "gen_tcp:recv"
> recv2(Socket, Bin) ->
>    case gen_tcp:recv(Socket, 0, ?TIMEOUT) of
>         {ok, B} ->
>             io:format(".", []),
>             recv(Socket, concat_binary([Bin, B]));
>         {error, closed} ->
>             {ok, Bin};
>        {error, timeout} ->
>             {error, {socket, timeout}};
>         Other ->
>             {error, {socket, Other}}
>     end.
>
>
> Which one is the best in my case (see below: fetch.erl)?
>
> Regards
> Zabrane
>
> Le 16 nov. 09 à 18:53, Chandru a écrit :
>
>> You are expecting the server to indicate end of response by closing the
>> connection, but because you specify HTTP/1.1 in the request, the server is
>> holding up your connection, and you are timing out. Try replacing HTTP/1.1
>> with HTTP/1.0 in your request, or parse the response to detect end of
>> response.
>>
>> cheers
>> Chandru
>>
>> 2009/11/16 zabrane Mikael <>
>>
>>> Hi List !
>>>
>>> New to Erlang, I'm trying to implement a simple URL fetcher.
>>> Here's my code (please, feel free to correct it if you find any bug or
>>> know
>>> a better approach):
>>>
>>>
>>>
>>> 8-----8-----8-----8-----8-----8-----8-----8-----8-----8-----8-----8-----8----
>>> -module(fetch).
>>>
>>> -export([url/1]).
>>>
>>> -define(TIMEOUT,    7000).
>>> -define(TCP_OPTS,   [binary, {packet, raw}, {nodelay, true},
>>>                   {active, true}]).
>>>
>>> url(Url) ->
>>>  {ok, _Tag, Host, Port} = split_url(Url),
>>>
>>>  Hdrs = [],
>>>  Request = ["GET ", Url, " HTTP/1.1\r\n", Hdrs, "\r\n\r\n"],
>>>
>>>  case catch gen_tcp:connect(Host, Port, ?TCP_OPTS) of
>>>    {'EXIT', Why} ->
>>>          {error, {socket_exit, Why}};
>>>      {error, Why} ->
>>>          {error, {socket_error, Why}};
>>>      {ok, Socket} ->
>>>          gen_tcp:send(Socket, list_to_binary(Request)),
>>>          recv(Socket, list_to_binary([]))
>>>  end.
>>>
>>> recv(Socket, Bin) ->
>>>  receive
>>>      {tcp, Socket, B} ->
>>>          io:format(".", []),
>>>          recv(Socket, concat_binary([Bin, B]));
>>>      {tcp_closed, Socket} ->
>>>          {ok, Bin};
>>>      Other ->
>>>          {error, {socket, Other}}
>>> after
>>>  ?TIMEOUT ->
>>>          {error, {socket, timeout}}
>>>  end.
>>>
>>>
>>> split_url([$h,$t,$t,$p,$:,$/,$/|T]) ->  split_url(http, T);
>>> split_url(_X)                       ->  {error, split_url}.
>>>
>>> split_url(Tag, X) ->
>>>  case string:chr(X, $:) of
>>>      0 ->
>>>          Port = 80,
>>>          case string:chr(X,$/) of
>>>              0 ->
>>>                  {ok, Tag, X, Port};
>>>              N ->
>>>                  Site = string:substr(X,1,N-1),
>>>                  {ok, Tag, Site, Port}
>>>          end;
>>>      N1 ->
>>>          case string:chr(X,$/) of
>>>              0 ->
>>>                  error;
>>>              N2 ->
>>>                  PortStr = string:substr(X,N1+1, N2-N1-1),
>>>                  case catch list_to_integer(PortStr) of
>>>                      {'EXIT', _} ->
>>>                          {error, port_number};
>>>                      Port ->
>>>                          Site = string:substr(X,1,N1-1),
>>>                          {ok, Tag, Site, Port}
>>>                  end
>>>          end
>>>  end.
>>>
>>>
>>>
>>> 8-----8-----8-----8-----8-----8-----8-----8-----8-----8-----8-----8-----8------
>>>
>>> When testing it, the receiving socket gets very very slow:
>>> $ erl
>>> 1> c(fetch).
>>> 2> Bin = fetch:url("http://www.google.com").
>>> ......{error,{socket,timeout}}
>>>
>>> Am I missing something?
>>> What I like to get at the end is a very fast fetcher. Any hint?
>>>
>>> Regards
>>> Zabrane
>>>
>
>
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