[erlang-bugs] Weird behaviour of gen_tcp:send/2 and erlang:port_command/3 with nosuspend to the same port on R16B03
Vasily Demidenok
define.null@REDACTED
Mon Jul 28 11:16:56 CEST 2014
Hello again
The friend of mine faced with the same problem. He assumes it's not tcp
driver related problem, but port internals changes made in R16 release.
Downgrading to R15B03 fixed their problem. +n options with d/s/a params
also did not help.
The app to reproduce bug:
https://github.com/kudryashov-sv/ErlangCPort.git
2014-07-05 0:47 GMT+04:00 Vasily Demidenok <define.null@REDACTED>:
> The problem remains even if only erlang:port_command/3 with nosuspend
> option is used. (no calls for gen_tcp:send and many processes write to the
> same socket)
>
>
> 2014-06-24 21:28 GMT+04:00 Lukas Larsson <lukas@REDACTED>:
>
> Hello,
>>
>> I was able to reproduce your testcase after removing all the "msg"
>> printouts and starting a couple of clients at the same time. It seems that
>> the sockets are hitting the high_msgq_watermark limit and then as data gets
>> flushes they are not set to run again. I'll see if I can dig out what it is
>> that is causing this behavior.
>>
>> Lukas
>>
>> On 24/06/14 16:56, Vasily Demidenok wrote:
>>
>> Hello list, we faced with some gen_tcp related problems after switch
>> from erlang R15B03 to R16B03-01
>>
>> The problem is as following: When server produce data faster then
>> consumer can handle, after
>> the out server's buffers are full and incoming client's buffers are full
>> gen_tcp:send/2 call on server side blocks forever in
>> erts_internal:port_command/3. After this, even when client consumes all
>> the data and the buffers are empty server process remains to be
>> suspended in that call
>>
>> This problem does not occur always, but quite often.
>>
>> Some details on implementation are below, I also shrink the example to
>> this small app so you can check the code:
>> https://github.com/define-null/tcp_failing_ex
>>
>> Server is implemented in such a way, that it listen on 8899 port, then
>> when client connect to it spawn main srv process and plenty of workers,
>> which start to write to this port after client send some special msg. The
>> main process is responsible for commands from the client and send responses
>> via gen_tcp:send/2, while workers try to write some stream data to the
>> client and use erang:port_command with no-suspend. So workers send only
>> up-to-date data, dropping any in case client is slow.
>>
>> The behaviour which we see is as following:
>> At first phase producer fills OS and erlang driver's buffers. Consumer
>> read data as it arrives and server drop data which it cannot send. So we
>> see buffer size growing on both side out queue of the server and in queue
>> of the client respectively
>>
>> After some moment in time, i guess when the buffers are completely
>> filled, server try respond to
>> ping message of the client, using gen_tcp:send/2 call. After that, it
>> blocks there forever, even after client consumes all the messages. The
>> situation does not change and the srv process remains in the suspended
>> state, while it's incoming buffer begins to grow when client send more ping
>> messages.
>>
>> Below is the output on the system with two slow clients, where for the
>> first client server's process is already blocked in gen_tcp:send/2 call,
>> while the second is served well.
>>
>> Every 2.0s: netstat -al | grep 8899
>>
>> Tue Jun 24 16:34:51 2014
>>
>> tcp4 36 0 localhost.8899 localhost.63263
>> ESTABLISHED
>> tcp4 0 0 localhost.63263 localhost.8899
>> ESTABLISHED
>> tcp4 0 130990 localhost.8899 localhost.63257
>> ESTABLISHED
>> tcp4 619190 0 localhost.63257 localhost.8899
>> ESTABLISHED
>> tcp4 0 0 *.8899 *.* LISTEN
>>
>> This is the output for the client process from github example, where we
>> see that
>> after send operation (ping msg) no incoming msg come any more.
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,28}}: msg
>> {{2014,6,24},{16,33,48}} before send
>> {{2014,6,24},{16,33,48}} after send ok
>> {{2014,6,24},{16,34,9}} before send
>> {{2014,6,24},{16,34,9}} after send ok
>> {{2014,6,24},{16,34,30}} before send
>> {{2014,6,24},{16,34,30}} after send ok
>> {{2014,6,24},{16,34,51}} before send
>> {{2014,6,24},{16,34,51}} after send ok
>> {{2014,6,24},{16,35,12}} before send
>> ....
>>
>> Server blocked process output:
>>
>> {{2014,6,24},{16,33,21}}: <0.95.0> ping
>> {{2014,6,24},{16,33,21}} bsend: <0.95.0>
>> {{2014,6,24},{16,33,21}} asend: <0.95.0> ok
>> {{2014,6,24},{16,33,48}}: <0.95.0> ping
>> {{2014,6,24},{16,33,48}} bsend: <0.95.0>
>> %% (no asend message after it)
>>
>> (tcp_failing_node@REDACTED)1> erlang:process_info(pid(0,95,0)).
>> [{current_function,{erts_internal,port_command,3}},
>> {initial_call,{proc_lib,init_p,5}},
>>
>> Bug is not always reproducible, but occurs quite often. The problem is
>> that even
>> after server's out buffers are empty data does not arrive to the client,
>> and incoming buffer grow
>> as client send ping messages to the server. (So erlang:port_command/3
>> with no-suspend always return false
>> when another main server process for this connection is suspended in
>> gen_tcp:send/2)
>>
>> And then it's getting only worse as already mentioned
>>
>> Every 2.0s: netstat -al | grep 8899
>>
>> Tue Jun 24 16:56:59 2014
>>
>> tcp4 804 0 localhost.8899 localhost.63263
>> ESTABLISHED
>> tcp4 0 0 localhost.63263 localhost.8899
>> ESTABLISHED
>> tcp4 0 0 localhost.8899 localhost.63257
>> ESTABLISHED
>> tcp4 0 0 localhost.63257 localhost.8899
>> ESTABLISHED
>> tcp4 0 0 *.8899 *.* LISTEN
>>
>> We faced with this after switching to R16B03 from R15B03, I know there
>> were some changes in port_command handling, i guess why we got such
>> behaviour?
>>
>>
>> _______________________________________________
>> erlang-bugs mailing listerlang-bugs@REDACTED://erlang.org/mailman/listinfo/erlang-bugs
>>
>>
>>
>
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