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CPU Metrics

CPU Load

Recall: Linux Process State

From man 1 ps:

D    uninterruptible sleep (usually IO)
R    running or runnable (on run queue)
S    interruptible sleep (waiting for an event to complete)
T    stopped, either by a job control signal or because it is being traced
Z    defunct ("zombie") process, terminated but not reaped by its parent

Task CL1: CPU Load

  1. What is the Load Average metric? Use the Linux Process States and man 5 proc (search for loadavg)

  2. Start the disk stress script (NOTE: avoid running it on your own SSD):

    scripts/disk/writer.sh

  3. Run the following command and look at the Load values for about a minute until ldavg-1 stabilizes:

    sar -q 1 100
    1. What is the writing speed of our script (ignore the first value, this is EBS General Purpose IOPS Burst)?
    2. What is the current Load Average? Why? Which processes contribute to this number?
    3. What are CPU %user, %IO-wait and %idle?

  4. While the previous script is running, start a single CPU stress:

    stress -c 1 -t 3600

    Wait another minute, and answer the questions above again.

  5. Stop all the scripts

Discussion

  • Why are processes waiting for IO included in the Load Average?
  • Assuming we have 1 CPU core and Load of 5, is our CPU core on 100% utilization?
  • How can we know if load is going up or down?
  • Does a load average of 70 indicate a problem?

Tools

  • Most tools use /proc/loadavg to fetch Load Average and run queue information.
  • Have a look at /proc/pressure/cpu for new PSI metrics
  • To get a percentage over a specific interval of time, you can use:
    • sar -q <interval> <count>
      • -q queue length and load averages
    • or simply uptime
  • Use eBPF based tools to observe the run queue: runqlat and runqlen

Next: Context Switches