NAME

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

SYNOPSIS

       tcpsynbl

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.

EXAMPLES

       Show the TCP SYN backlog as a histogram.
              # tcpsynbl

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)