bcc-tcpsynbl(8)

1tcpsynbl(8)                 System Manager's Manual                tcpsynbl(8)
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NAME

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

9       tcpsynbl [-4 | -6]
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DESCRIPTION

12       This  tool  shows the TCP SYN backlog size during SYN arrival as a his‐
13       togram.  This lets you see how close your applications are  to  hitting
14       the  backlog  limit  and dropping SYNs (causing performance issues with
15       SYN retransmits), and is a measure of  workload  saturation.  The  his‐
16       togram  shown  is  measured at the time of SYN received, and a separate
17       histogram is shown for each backlog limit.
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19       This    works    by    tracing    the    tcp_v4_syn_recv_sock()     and
20       tcp_v6_syn_recv_sock()  kernel functions using dynamic instrumentation.
21       Since these functions may change in future kernels, this tool may  need
22       maintenance to keep working.
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24       Since this uses BPF, only the root user can use this tool.
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REQUIREMENTS

27       CONFIG_BPF and BCC.
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OPTIONS

30       -h     Print usage message.
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32       -4     Trace IPv4 family only.
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34       -6     Trace IPv6 family only.
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EXAMPLES

37       Show the TCP SYN backlog as a histogram.
38              # tcpsynbl
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40       Trace IPv4 family only:
41              # tcpsynbl -4
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43       Trace IPv6 family only:
44              # tcpsynbl -6
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FIELDS

47       backlog
48              The backlog size when a SYN was received.
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50       count  The number of times this backlog size was encountered.
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52       distribution
53              An ASCII visualization of the count column.
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OVERHEAD

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

60       This originated as a bpftrace  tool  from  the  book  "BPF  Performance
61       Tools", published by Addison Wesley (2019):
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63              http://www.brendangregg.com/bpf-performance-tools-book.html
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65       See the book for more documentation on this tool.
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67       This version is in the BCC repository:
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69              https://github.com/iovisor/bcc
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71       Also  look  in  the bcc distribution for a companion _examples.txt file
72       containing example usage, output, and commentary for this tool.
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OS

75       Linux
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STABILITY

78       Unstable - in development.
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AUTHOR

81       Brendan Gregg
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