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CPU Metrics

Context Switches

Recall: Linux Process Context Switches

A mechanism to store current process state ie. Registers, Memory maps, Kernel structs (eg. TSS in 32bit), and load another (or a new one). Context switches are usually computationally expensive (although optimization exist), yet inevitable. For example, they are used to allow multi-tasking (eg. preemption), and to switch between user and kernel modes.

Task CS1: Context Switches

  1. Execute vmstat 2 and write down the current context switch rate (cs field)
  2. Raise that number by executing stress -i 10
    1. What is the current context switch rate?
    2. What is causing this rate? Multi-tasking? Interrupts? Switches between kernel and user modes?
    3. Kill the stress command, and watch the rate drop
  3. Now let's see how a high context switch rate affects a dummy application
    1. Run the dummy application perf stat -e cs python scripts/cpu/dummy_app.py (which calls a dummy function 1000 times, and prints it's runtime percentile)
    2. Write the current CPU usage, the application percentiles and context switch rate
    3. In the same session, raise the context switch rate using stress -i 10 & and re-run the dummy application. Write the current CPU usage, the application percentiles and context switch rate.
    4. Describe the change in the percentiles. Did the high context switch rate affect most of foo() runs (ie. the 50th percentile)? If not, why?
  4. Finally, observe the behaviour when running stress in a different scheduling task group
    1. Move one of your sessions to a different cgroup sudo mkdir /sys/fs/cgroup/cpu/a; echo $$ | sudo tee /sys/fs/cgroup/cpu/a/tasks
    2. Run stress again stress -i 10 or stress -c 10
    3. Compare the CPU usage to 3.iii (it should be roughly the same) and compare the context switch rate (which should be the same)
    4. Re-run the dummy application and describe the change in the percentiles (and process context switch) vs 3.iv

Discussion

  • Can performance measurements on a staging environment truly estimate performance on production?
  • Why did we run the stress command and our dummy application in the same session?

Tools

  • Most tools use /proc/vmstat to fetch global context switching information (ctxt field), and /proc/[PID]/status for process specific information (voluntary_context_switches and nonvoluntary_context_switches fields)
  • From the command-line you can use
    • vmstat <delay> <count> for global information
    • pidstat -w -p <pid> <delay> <count> for process specific information

Next: Memory Usage