tcpsynbl.bt(8)

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

6       tcpsynbl.bt  -  Show  the  TCP  SYN  backlog  as a histogram. Uses bpf‐
7       trace/eBPF.
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SYNOPSIS

10       tcpsynbl
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DESCRIPTION

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

28       CONFIG_BPF and bpftrace.
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EXAMPLES

31       Show the TCP SYN backlog as a histogram.
32              # tcpsynbl.bt
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FIELDS

35       backlog
36              The backlog size when a SYN was received.
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38       count  The number of times this backlog size was encountered.
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40       distribution
41              An ASCII visualization of the count column.
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OVERHEAD

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

48       This  tool  originated from the book "BPF Performance Tools", published
49       by Addison Wesley (2019):
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51              http://www.brendangregg.com/bpf-performance-tools-book.html
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53       See the book for more documentation on this tool.
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55       This version is in the bpftrace repository:
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57              https://github.com/iovisor/bpftrace
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59       Also look in the bpftrace distribution for  a  companion  _examples.txt
60       file containing example usage, output, and commentary for this tool.
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OS

63       Linux
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STABILITY

66       Unstable - in development.
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AUTHOR

69       Brendan Gregg
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