I have an application that sends lot of traffic over an UDP socket, every packet is sent on 2 interfaces: enp2s0 (1Gbit ethernet device) and enx00808a8eba78 (100Mbit usb-ethernet device).
The maximum socket send buffer is the default (212992 bytes), and it is full most of the time when the traffic is running:

root@punk:~# netstat -a
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
udp        0 211968 *   

Data in qdisc queue of the two interfaces is about 40k:

root@punk:~# tc -s qdisc show dev enp2s0
qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 1697909529 bytes 1136534 pkt (dropped 0, overlimits 0 requeues 12) 
 backlog 0b 0p requeues 12 

root@punk:~# tc -s qdisc show dev enx00808a8eba78
qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 1675952337 bytes 1121840 pkt (dropped 0, overlimits 0 requeues 55) 
 backlog 43326b 29p requeues 55 

Since 200k of data is pending in the socket but only 40k is queued in the second qdisc, I assume that the remaining 160k are pending inside the slow interface driver (enx00808a8eba78).

Is there a way to check how many packets (or data) is pending for transmission in a USB device or, more generically, in a network device?
Something like the number of DMA buffers ready for TX but not sent yet.

  • You would be far better off without the USB ethernet. Jan 28, 2019 at 17:54

1 Answer 1


Seems there isn't a way to retrieve the device queue length from userspace.

BTW, some details if someone is interested:

usbnet devices keep track of the queued TX packets using the field txq.qlen of the struct usbnet. Maximum TX queue length is defined by field tx_qlen of struct usbnet.

In my example I have 60 (tx_qlen) packets queued in the USB driver and (more or less) 30 packets in the qdisc, each one carrying 1500 bytes of data. Since socket buffer is calculated considering the skb->truesize (i.e. skb data + skb structure size), each packet is 2.3k:

2.3k * (60 + 30) ~= 200k

This confirm that 138k of the socket buffer are consumed by packets queued in the network driver, while 69k of the socket buffer are in the qdisc queue: there aren't other packets queued somewhere else in the kernel.

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