1tcpsynbl.bt(8) System Manager's Manual tcpsynbl.bt(8)
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6 tcpsynbl.bt - Show the TCP SYN backlog as a histogram. Uses bpf‐
7 trace/eBPF.
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10 tcpsynbl
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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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28 CONFIG_BPF and bpftrace.
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31 Show the TCP SYN backlog as a histogram.
32 # tcpsynbl.bt
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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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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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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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63 Linux
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66 Unstable - in development.
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69 Brendan Gregg
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72 tcptop(8)
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76USER COMMANDS 2019-07-03 tcpsynbl.bt(8)