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

# [NAME](#NAME)

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

# [SYNOPSIS](#SYNOPSIS)

**tcpsynbl**

# [DESCRIPTION](#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](#REQUIREMENTS)

CONFIG\_BPF and bpftrace.

# [EXAMPLES](#EXAMPLES)

[Show the TCP SYN backlog as a histogram.](#Show)

\# **tcpsynbl.bt**

# [FIELDS](#FIELDS)

[backlog](#backlog)

The backlog size when a SYN was received.

[count](#count)

The number of times this backlog size was encountered.

[distribution](#distribution)

An ASCII visualization of the count column.

# [OVERHEAD](#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](#SOURCE)

This tool originated 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 bpftrace repository:

https://github.com/bpftrace/bpftrace

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

# [OS](#OS)

Linux

# [STABILITY](#STABILITY)

Unstable - in development.

# [AUTHOR](#AUTHOR)

Brendan Gregg

# [SEE ALSO](#SEE_ALSO)

[tcptop(8)](/man/enterprise-packages/bpftrace/tcptop.8)

2019-07-03 USER COMMANDS

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Document: tcpsynbl.bt(8) from bpftrace 0.27.0-r1 (Chainguard OS)
Source: https://github.com/bpftrace/bpftrace