NAME

       tcpsynbl - Show the TCP SYN backlog as a histogram. Uses BCC/eBPF.

SYNOPSIS

       tcpsynbl [-4 | -6]

DESCRIPTION

       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), and is a measure of workload saturation. The histogram shown is measured at the time of SYN
       received, and a separate histogram is shown for each backlog limit.

       This works by tracing  the  tcp_v4_syn_recv_sock()  and  tcp_v6_syn_recv_sock()  kernel  functions  using
       dynamic  instrumentation.  Since  these  functions  may  change  in  future  kernels,  this tool may need
       maintenance to keep working.

       Since this uses BPF, only the root user can use this tool.

REQUIREMENTS

       CONFIG_BPF and BCC.

OPTIONS

       -h     Print usage message.

       -4     Trace IPv4 family only.

       -6     Trace IPv6 family only.

EXAMPLES

       Show the TCP SYN backlog as a histogram.
              # tcpsynbl

       Trace IPv4 family only:
              # tcpsynbl -4

       Trace IPv6 family only:
              # tcpsynbl -6

FIELDS

       backlog
              The backlog size when a SYN was received.

       count  The number of times this backlog size was encountered.

       distribution
              An ASCII visualization of the count column.

OVERHEAD

       Inbound SYNs should be relatively low compared to packets and other events, so the overhead of this  tool
       is expected to be negligible.

SOURCE

       This  originated  as  a  bpftrace tool from the book "BPF Performance Tools", published by Addison Wesley
       (2019):

              http://www.brendangregg.com/bpf-performance-tools-book.html

       See the book for more documentation on this tool.

       This version is in the BCC repository:

              https://github.com/iovisor/bcc

       Also look in the bcc distribution for a companion _examples.txt file containing  example  usage,  output,
       and commentary for this tool.

OS

       Linux

STABILITY

       Unstable - in development.

AUTHOR

       Brendan Gregg

SEE ALSO

       tcptop(8)

USER COMMANDS                                      2019-07-03                                        tcpsynbl(8)