Capturing on loopback device on Ubuntu LTS: 15 Mbit of traffic and "x dropped packets"

Asked Aug 12 '13 at 14:50

Active Oct 18 '15 at 08:35

Viewed 368 times

I am trying to capture about 15 Mbit of traffic on the loopback device on Ubuntu LTS using tcpdump, and I am seeing dropped packets.

The CPU is a freaking i7, and i have yet to see CPU usage go above 15% during the measurements i am trying to run. Load is negligible...

The capture is written to a file using -w.

I have tried the following (as suggested here: https://stackoverflow.com/a/7018039/55267 ):

Set /proc/sys/net/core/netdev_max_backlog and /proc/sys/net/core/netdev_budget to arbitarily high values. No effect.
-nn No effect
Tried changing txqueuelen of lo. No effect.
Tried using gulp. No effect.
- gulp instructed me to:
  - echo 4194304 > /proc/sys/net/core/rmem_max
  - echo 4194304 > /proc/sys/net/core/rmem_default
  - No effect....

Is there something special about the loopback device in regard to lossless captures? How can I do a lossless capture on lo?

edited May 23 '17 at 12:41

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asked Aug 12 '13 at 14:50

Tarnay Kálmán

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Are you sure you can write fast enough? What's the iowait like? – Tom O'Connor Aug 12 '13 at 14:56
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When you say that CPU usage is not "above 15%", is that for the entire system or a limit per-core? Depending on where you're looking, 15% could be a single core of a HT quad-core pegged. – chrylis -cautiouslyoptimistic- Aug 12 '13 at 14:57
@TomO'Connor I looked using `iostat -x 1`, and it never went above 3%. – Tarnay Kálmán Aug 12 '13 at 15:19
@chrylis Entire system. Actually tcpdump is running niced to -20. (Everything else has priority of -10 or more.) – Tarnay Kálmán Aug 12 '13 at 15:45
@TomO'Connor ALso, I have tried gulp which is supposed to help with write speed issues. It keeps a 100 Mb ringbuffer in memory by default. – Tarnay Kálmán Aug 12 '13 at 15:58
And there are no any funky iptables rules throttling traffic to loopback interface? – Janne Pikkarainen Aug 12 '13 at 16:02
@JannePikkarainen Hm, actually I do use netem to delay some traffic. – Tarnay Kálmán Aug 12 '13 at 16:12
@JannePikkarainen Non-delayed traffic is also getting lost. I will try getting rid of all tc stuff to see if it helps. – Tarnay Kálmán Aug 12 '13 at 16:18
@JannePikkarainen: Got rid of the throttling. No effect, still losing packets. (`tc filter show dev lo` outputs nothing now, and `sudo iptables --list` shows no rules) – Tarnay Kálmán Aug 12 '13 at 16:24
Oh well, that was worth a shot anyway. :) – Janne Pikkarainen Aug 12 '13 at 16:25
Yeah, thx for it. :) – Tarnay Kálmán Aug 12 '13 at 16:29
Want a real challenge? Try the same thing again with a realtime kernel, and renice the process to realtime priorities.. – Tom O'Connor Aug 12 '13 at 17:34
https://help.ubuntu.com/community/UbuntuStudio/RealTimeKernel – Tom O'Connor Aug 12 '13 at 17:39
Why would that be a real challenge? – Tarnay Kálmán Aug 12 '13 at 23:02

Capturing on loopback device on Ubuntu LTS: 15 Mbit of traffic and "x dropped packets"

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