The missing UDP packets

[Andreas Schultz]

Am Di., 4. Feb. 2020 um 14:35 Uhr schrieb Alexander Petrovsky <
askjuise@REDACTED>:

> Paul, thank you for your quick response, it seems like I'm was not
> accurate enough in my previous letter )
>
> I do not wonder about drops, I know that drops could occur in buffers in
> network interface, in kernel buffers and also in buffers in VM. In my case
> there are not drops in kernel space accordingly to netstat -S, ss -anump, cat
> /proc/net/snmp | grep Udp, cat /proc/net/udp; also it could be the bug in
> my code, which I can't find by some reason ))
>
> Please, pay attention that after I'm trying to debug this behavior with
> strace/gdb the problem has gone. I'm interesting, how could I find/observer
> is there any drops in VM buffers or so... ?
>

One thing you could try is to use systemtap instead of strace/gdb to
monitor the UDP socket. It has much less impact on runtime behavior and
might provide a clue as to whether the data is actually read from socket.

Also, have you tried using the new socket module instead of gen_udp?

Andreas


> вт, 4 февр. 2020 г. в 16:07, Paul Peregud <paulperegud@REDACTED>:
>
>> AFAIK UDP packet can be dropped by anyone. By intermediate nodes and
>> by your kernel. I would just assume that they can be dropped by your
>> BEAM VM too. I also don't think that "it's always the second packet"
>> behavior is strange. Play around with kernel buffers sizes and you
>> will likely observe some change.
>>
>> Assumption that UDP packet that was sent will be delivered is wrong.
>>
>> On Tue, Feb 4, 2020 at 8:32 AM Alexander Petrovsky <askjuise@REDACTED>
>> wrote:
>> >
>> > Hello!
>> >
>> > The first of all, my erlang version OTP-21.3 (without any minor patches)
>> >
>> > I've got the strange floating problem with UDP to receive, which I
>> couldn't debug by myself. Let me describe briefly what I have: There is UDP
>> socket which used for send packets to many peers and receives responses
>> from them (rate ~1pps):
>> >
>> > send request: SIP options
>> > receive a response: 100
>> > receive a response: 200
>> >
>> > In erlang it's implemented like gen_server which receives messages, the
>> sockets options are: binary, {active, once}.
>> >
>> > Every time, when the message is received from the mailbox, code try to
>> do some job, after that try to handle message bursts by calling in the loop
>> (by example 5 times):
>> > gen_udp:recv(Socket, 0, 0), bursts and after that set the socket option
>> {active, once} back.
>> >
>> > So, here I'm observing floating strange behavior, from time to time the
>> second!! and always the second!! response is absent. I see this UDP missing
>> packets in tcpdump, I don't see any drops in UDP stack at all.
>> >
>> > When I'm trying to add some logs into my erlang code, the problem
>> disappears, when I'm run strace to recvfrom syscall the problem
>> disappears... I't try to debug VM with gdb and make some breakpoints on
>> sock_recvfrom in inet_drv.c (it's OTP-21.3), but the problem disappears (((
>> >
>> > I can't find the problem root cause, is it in my code or is it in VM?
>> Maybe someone could help me with that?
>> >
>> > --
>> > Петровский Александр / Alexander Petrovsky,
>> >
>> > Skype: askjuise
>> > Phone: +7 931 9877991
>> >
>>
>>
>> --
>> Best regards,
>> Paul Peregud
>> +48602112091
>>
>
>
> --
> Петровский Александр / Alexander Petrovsky,
>
> Skype: askjuise
> Phone: +7 931 9877991
>
>

-- 

Andreas Schultz

-- 

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