[erlang-questions] how to flush a gen_tcp socket on close?

Tony Rogvall <>
Thu Apr 5 19:39:17 CEST 2012


Hi!

I want to add some clues to this puzzle.

First of all the right way: send, shutdown wait for close is not working here as would have been 
my first answer :-) 
Then I moved the subscription code to just wait for the empty queue event, and NOT do the timeout. 
This worked well. My interpretation is that the reader is so slow, so that the write side 
"write ready" is not signaled and no more data goes down to kernel space (probably some threshold.)
This will keep the pending data at the same level, and hence timeout.

My scheme looked like:

	fill output buffer 1K at a time until send_pend > 0 
	start reader
	subscribe for empty_q  (code from prim inet)
	wait for empty_q
	send shutdown
	wait for close

So how would a workaround look like ? 

One way is to make sure that the kernel buffer is smaller (now I can send 160K). Use what ever size that
make sense with the knowledge of the inet built in time out timer (5 secs)

An other way to fix this could be to try to add kernel buffer space to send_pend but that is 
pretty os dependent :-)

/Tony



On 5 apr 2012, at 00:13, Matthias Lang wrote:

> On Tuesday, April 03, Andreas Schultz wrote:
> 
>> In my case the receiver is to slow to process all the data, sender
>> does 10k packets of 1k size, the receiver only gets the first 2000
>> packets. It might well be that I hit the close timeout and inet
>> discards the rest of the send queue.
> 
> Ok, that's something to go on.
> 
> I had a go at reproducing what you're doing based on that description.
> I see something unexpected which seems similar to what you reported.
> 
> The output of my program (at the bottom of this mail) running on R15B is:
> 
>   30> as_tcp:go().
>   calling close on TX at {23,35,35}
>   read 1000 octets
>   read 1000 octets
>   read 1000 octets
>   read 1000 octets
>   read 1000 octets
>   read 1000 octets
>   read 1000 octets
>   read 1000 octets
>   read 1000 octets
>   close returned at {23,35,45}, 10002ms later
>   ok
>   read 1000 octets
>   31>
>   =ERROR REPORT==== 4-Apr-2012::23:35:45 ===
>   Error in process <0.120.0> with exit value: {{badmatch,{error,closed}},[{as_tcp,slow_read,1,[{file,"as_tcp.erl"},{line,31}]}]}
> 
> I had a bit of a dig in prim_inet.erl. It sounds like you've looked
> there too. That code looks like it's intended to loop 'forever' trying
> to send the queued data, as long as some progress is made every so often
> (always sends at least something in every 5s timeout period). But running
> my program suggests that isn't happening as intended.
> 
> Instrumenting prim_inet, it looks like the {subs_empty_out_q,0 } message
> is coming up from inet_drv.c even though the queue isn't empty. Looking
> in there, I noticed this:
> 
>   static void tcp_inet_flush(ErlDrvData e)
>   {
>       tcp_descriptor* desc = (tcp_descriptor*)e;
>       if (!(desc->inet.event_mask & FD_WRITE)) {
>   	/* Discard send queue to avoid hanging port (OTP-7615) */
>   	tcp_clear_output(desc);
>       }
>   }
> 
> but I think that change was introduced sometime after R11B-5, and
> R11B-5 fails the same way. So probably a red herring.
> 
> Out of time looking at this for now.
> 
>> The fix should be simple, limit the send queue size.
> 
> To what?
> 
> Zero seems to be the only value that will work even for arbitrarily slow
> clients. And that defeats the point of having a send queue.
> 
> ---
> 
> A likely _workaround_ is to call
> 
>  inet:getstat(Tx, [send_pend])
> 
> if the answer is zero, then you know that you can call close() without
> _erlang_ tossing data. I haven't tried this.
> 
> ---
> 
> It's late, I might have outsmarted myself, but my current feeling is
> that erlang is quietly tossing data and it shouldn't be.
> 
> Waiting for as long as it takes in close() seems like the right thing,
> though Per might disagree. Waiting for N seconds in close() and then
> returning an error if the queue didn't empty would also be better than
> just quietly tossing it.
> 
> (And: yes, I know, application-level ACKs would avoid this
> problem. But I'm not quite ready to say that this problem can't be
> fixed.)
> 
> Matt
> 
> ----------------------------------------------------------------------
> %% Throwaway module: attempt to reproduce TCP problem reported to
> %% erlang-questions 2012-04-02 by Andreas Schultz.
> %%
> %% He elaborated on 2012-04-04: gen_tcp drops data when close is called
> %% with data buffered if the receiver is sufficiently slow.
> -module(as_tcp).
> -export([go/0]).
> 
> go() ->
>    {ok, L} = gen_tcp:listen(0, [{active, false}, binary]),
>    {ok, Portno} = inet:port(L),
>    {ok, Tx} = gen_tcp:connect(localhost, Portno, []),
>    {ok, Rx} = gen_tcp:accept(L),
>    ok = gen_tcp:close(L),
> 
>    One_hundred_k = list_to_binary(lists:duplicate(100000, 0)),
>    ok = gen_tcp:send(Tx, [lists:duplicate(40, One_hundred_k), "end of data"]),
> 
>    spawn(fun() -> slow_read(Rx) end),
> 
>    io:fwrite("calling close on TX at ~p\n", [time()]),
>    Before = now(),
>    ok = gen_tcp:close(Tx),
>    After = now(),
>    io:fwrite("close returned at ~p, ~pms later\n",
> 	      [time(), timer:now_diff(After, Before) div 1000]),
> 
>    ok = gen_tcp:close(Rx).
> 
> slow_read(Rx) ->
>    {ok, _Bin} = gen_tcp:recv(Rx, 10000),
>    timer:sleep(1000),
>    io:fwrite("read 1000 octets\n"),
>    slow_read(Rx).
> 
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