Floating point exception in slowdownSndPeriod() with Ubuntu NetEm tool

[stevenwff]

I'm testing the performance of srt-file-transmit , when I used the tc & netem tool to get an environment with 1% packet loss, I came across a "Floating point exception" with srt-file-transmit.

Related Info:
SRT version : v1.4.0, Commit fe2858d
OS : Client - Ubuntu 18.04 64bits, Server - Ubuntu 16.04 64btis
Network : 1Gbps Lan
NetEm command : tc qdisc add dev eth0 root netem loss 1%
srt command : ./srt-file-transmit -s:1000 -pf:csv -statsout:statsnd.csv -logfile:log.txt -logfa:general file:///home/lzx/srt/4K2013.mp4 srt://10.48.40.251:7708

I used the netem command to set up the network and then used the srt command to send a file ~200MB. I exchange the client and server, and it made no difference.

The exception was located in "FileCC::slowdownSndPeriod", Line 497:

      const int pktsInFlight = m_parent->RTT() / m_dPktSndPeriod;
      const int numPktsLost = m_parent->sndLossLength();
      const int lost_pcent_x10 = (numPktsLost * 1000) / pktsInFlight;

It supposed pktsInFlight wouldn't be 0, but actually the in this case, m_parent->RTT() kept decreasing and m_dPktSndPeriod kept increasing, then m_parent->RTT() < m_dPktSndPeriod and the FPE came out.

I insert some test code to show the process. RTT stands for m_parent->RTT() , PSP for m_dPktSndPeriod and loss size for losslist_size.

Target connected (caller)
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 1.000000 
RTT: 95733, PSP: 11.768721 
pktsInFlight: 8134 
m_dPktSndPeriod KEPT 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 11.768721 
RTT: 95733, PSP: 11.768721 
pktsInFlight: 8134 
m_dPktSndPeriod KEPT 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 11.768721 
RTT: 95733, PSP: 11.768721 
pktsInFlight: 8134 
m_dPktSndPeriod KEPT 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 11.768721 
RTT: 95733, PSP: 11.768721 
pktsInFlight: 8134 
m_dPktSndPeriod KEPT 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 11.768721 
RTT: 95733, PSP: 11.768721 
pktsInFlight: 8134 
m_dPktSndPeriod KEPT 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 11.768721 
RTT: 92166, PSP: 11.768721 
pktsInFlight: 7831 
m_dPktSndPeriod KEPT 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 11.768721 
RTT: 92166, PSP: 11.768721 
pktsInFlight: 7831 
m_dPktSndPeriod KEPT

.............................................
.............................................

#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 283.000000 
RTT: 364, PSP: 283.000000 
pktsInFlight: 1 
m_dPktSndPeriod AFTER: 292.000000 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 291.914761 
RTT: 362, PSP: 291.914761 
pktsInFlight: 1 
m_dPktSndPeriod KEPT 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 291.065099 
RTT: 359, PSP: 291.065099 
pktsInFlight: 1 
m_dPktSndPeriod AFTER: 300.000000 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 300.000000 
RTT: 357, PSP: 300.000000 
pktsInFlight: 1 
m_dPktSndPeriod AFTER: 309.000000 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 309.000000 
RTT: 353, PSP: 309.000000 
pktsInFlight: 1 
m_dPktSndPeriod KEPT 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 307.102109 
RTT: 344, PSP: 307.102109 
pktsInFlight: 1 
m_dPktSndPeriod AFTER: 317.000000 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 316.899543 
RTT: 338, PSP: 316.899543 
pktsInFlight: 1 
m_dPktSndPeriod AFTER: 327.000000 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 327.000000 
RTT: 338, PSP: 327.000000 
pktsInFlight: 1 
m_dPktSndPeriod AFTER: 337.000000 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 336.773015 
RTT: 342, PSP: 336.773015 
pktsInFlight: 1 
m_dPktSndPeriod AFTER: 347.000000 
#1 =============Running..........
Loss size: 1
m_dPktSndPeriod BEFORE: 347.000000 
RTT: 342, PSP: 347.000000 
pktsInFlight: 0 
Floating point exception (core dumped)

It looks a little weird because as i tried the same srt command in a real network with packet loss, everything seems all right. I tried to get the log file but only one-line connection information was logged.

Any idea why the problem happened? Thanks!

The stats file was attached.

statsnd.zip

[ethouris]

I smell division by 0. Might even be that this pktsInFlight was the divisor. I don't know where these above logs come from, but if this pktsInFlight represents what is defined in congctl.cpp:495 line, this would definitely do it in this case:

        const int lost_pcent_x10 = (numPktsLost * 1000) / pktsInFlight;

I'll fix it.

[ethouris]

Fixed by #888

[stevenwff]

I modified slowdownSndPeriod() and generated the logs above , to see why pktsInFlight kept decreasing to zero, and it's excactly the pktsInFlight defined in congctl.cpp:495.
If it wouldn't affect the performance, #888 surely fix it.
Thanks for quick response!

[ethouris]

Thanks; I wouldn't have quickly an env for testing it, just was 99.9% sure that this will exactly fix it. Therefore thanks for making it 100% certainty.