[erlang-bugs] 回复： Weird behaviour of gen_tcp:send/2 anderlang:port_command/3 with nosuspend to the same port on R16B03
o鸡蛋黄o
53817681@REDACTED
Mon Jul 28 11:41:54 CEST 2014
Dear,
a bug in erlang vm sourcecode , port_get_data_1 process arg is NULL , I had fixed it,thanks.
------------------ 原始邮件 ------------------
发件人: "Vasily Demidenok";<define.null@REDACTED>;
发送时间: 2014年7月28日(星期一) 下午5:16
收件人: "Lukas Larsson"<lukas@REDACTED>;
抄送: "erlang-bugs"<erlang-bugs@REDACTED>;
主题: Re: [erlang-bugs] Weird behaviour of gen_tcp:send/2 anderlang:port_command/3 with nosuspend to the same port on R16B03
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?
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