tcpsynbl_example.txt

Demonstrations of tcpsynbl, the Linux bpftrace/eBPF version.


This tool shows the TCP SYN backlog size during SYN arrival as a histogram.
This lets you see how close your applications are to hitting the backlog limit
and dropping SYNs (causing performance issues with SYN retransmits). For
example:

# ./tcpsynbl.bt 
Attaching 4 probes...
Tracing SYN backlog size. Ctrl-C to end.
^C
@backlog[backlog limit]: histogram of backlog size


@backlog[500]: 
[0]                 2266 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[1]                    3 |                                                    |
[2, 4)                 1 |                                                    |

This output shows that for the backlog limit of 500, there were 2266 SYN
arrivals where the backlog was zero, three where the backlog was one, and
one where the backlog was either two or three. This indicates that we are
nowhere near this limit.