cpuunclaimed man page

cpuunclaimed ā€” Sample CPU run queues and calculate unclaimed idle CPU. Uses Linux eBPF/bcc.

Synopsis

cpuunclaimed [-T] [-j] [-J] [interval [count]]

Description

This tool samples the length of the run queues and determine when there are idle CPUs, yet queued threads waiting their turn. It reports the amount of idle (yet unclaimed by waiting threads) CPU as a system-wide percentage.

This situation can happen for a number of reasons:

An unclaimed idle of < 1% is likely to be CPU affinity, and not usually a cause for concern. By leaving the CPU idle, overall throughput of the system may be improved. This tool is best for identifying larger issues, > 2%, due to the coarseness of its 99 Hertz samples.

This is an experimental tool that currently works by use of sampling to keep overheads low. Tool assumptions:

If this identifies unclaimed CPU, you can double check it by dumping raw samples (-j), as well as using other tracing tools to instrument scheduler events (although this latter approach has much higher overhead).

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

Requirements

CONFIG_BPF and bcc.

Examples

Sample and calculate unclaimed idle CPUs, output every 1 second (default:

# cpuunclaimed

Print 5 second summaries, 10 times:

# cpuunclaimed 5 10

Print 1 second summaries with timestamps:

# cpuunclaimed -T 1

Raw dump of all samples (verbose), as comma-separated values:

# cpuunclaimed -j

Fields

%CPU

CPU utilization as a system-wide percentage.

unclaimed idle

Percentage of CPU resources that were idle when work was queued on other CPUs, as a system-wide percentage.

TIME

Time (HH:MM:SS)

TIMESTAMP_ns

Timestamp, nanoseconds.

CPU#

CPU ID.

OFFSET_ns_CPU#

Time offset that a sample fired within a sample group for this CPU.

Overhead

The overhead is expected to be low/negligible as this tool uses sampling at 99 Hertz (on all CPUs), which has a fixed and low cost, rather than sampling every scheduler event as many other approaches use (which can involve instrumenting millions of events per second). Sampled CPUs, run queue lengths, and timestamps are written to ring buffers that are periodically read by user space for reporting. Measure overhead in a test environment.

Source

This is from bcc.

https://github.com/iovisor/bcc

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

OS

Linux

Stability

Unstable - in development.

Author

Brendan Gregg

See Also

runqlen(8)

Info

2016-12-21 USER COMMANDS