NAME¶

biolatency.bt - Block I/O latency as a histogram. Uses bpftrace/eBPF.

SYNOPSIS¶

biolatency.bt

DESCRIPTION¶

This tool summarizes time (latency) spent in block device I/O (disk I/O) as a power-of-2 histogram. This allows the distribution to be studied, including modes and outliers. There are often two modes, one for device cache hits and one for cache misses, which can be shown by this tool. Latency outliers will also be shown.

The original tool, which is retained as "biolatency-kp.bt", currently works by dynamic tracing of the blk_account*() kernel functions, which will need updating to match any changes to these functions in future kernels versions.

The updated version of the tool utilizes tracepoints instead of kprobes so that it can be compatible with a wide range of kernel versions.

Since this uses BPF, only the root user can use this tool.

REQUIREMENTS¶

CONFIG_BPF and bpftrace.

EXAMPLES¶

Trace block device I/O (disk I/O), and print a latency histogram on Ctrl-C:

# biolatency.bt

FIELDS¶

1st, 2nd

This is a range of latency, in microseconds (shown in "[...)" set notation).

3rd

A column showing the count of operations in this range.

4th

This is an ASCII histogram representing the count column.

OVERHEAD¶

Since block device I/O usually has a relatively low frequency (< 10,000/s), the overhead for this tool is expected to be negligible. For high IOPS storage systems, test and quantify before use.

SOURCE¶

This is from bpftrace.

https://github.com/iovisor/bpftrace

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

This is a bpftrace version of the bcc tool of the same name. The bcc tool may provide more options and customizations.

https://github.com/iovisor/bcc

OS¶

Linux

STABILITY¶

Unstable - in development.

AUTHOR¶

Brendan Gregg

SEE ALSO¶

biosnoop.bt(8)