High CPU usage and Context Switching

A high rate of context switching in Linux can indicate that your system is spending a lot of time switching between processes or threads, which might lead to performance issues like CPU bottlenecks or inefficient resource usage. Troubleshooting this involves identifying the cause and taking steps to mitigate it. Here’s a step-by-step guide:

1. Confirm High Context Switching

• Use tools like vmstat or sar to measure context switching rates:

Look at the cs column (context switches per second). A "high" rate depends on your system’s workload—hundreds or thousands per second might be normal for a busy server, but excessive rates (e.g., tens of thousands) could signal a problem.

Alternatively, with sar (if sysstat is installed):

• sar -w 5

2 Check CPU Usage

• High context switching often correlates with CPU contention. Use top or htop to see overall CPU usage and identify if the system is overloaded:

Look at the %us (user), %sy (system), and %id (idle) columns. Low idle time suggests the CPU is busy.

Check the number of runnable tasks in vmstat (column r under "procs"). If this exceeds the number of CPU cores, processes are waiting, causing more context switches.

3 Identify Culprit Processes

• Use pidstat (from sysstat) to pinpoint processes causing context switches:

pidstat -w 1

• cswch/s: Voluntary context switches (e.g., process waiting for I/O).

• nvcswch/s: Non-voluntary context switches (e.g., CPU preempting the process). High values in either column can indicate specific process behavior.

Cross-reference with top or ps to see what those processes are doing:

ps -aux | grep <pid>

4. Analyze System Load

Check the system load average with uptime or top:

uptime

A load average much higher than the number of CPU cores (viewable with nproc) suggests too many tasks are competing for CPU time, increasing context switches.

5. Investigate Interrupts

• Excessive interrupts (e.g., from network or disk I/O) can drive up context switching. Check interrupt activity with:

cat /proc/interrupts

Or use vmstat again and look at the in column (interrupts per second). If this is high, investigate hardware or driver issues (e.g., a chatty NIC).

6. Examine Scheduling and Threading

• If a single process has many threads, it could be thrashing the scheduler. Use ps to check thread count:

ps -eL | grep <process_name> | wc -l

Check the scheduling policy with chrt:

chrt -p <pid>

Real-time policies (e.g., SCHED_FIFO) might cause excessive preemptions.

7. Look at I/O Bottlenecks

• High voluntary context switches often mean processes are waiting for I/O. Use iostat or iotop to check disk activity:

iostat -x 1

High %iowait in top or vmstat confirms this. If I/O is the issue, optimize disk usage or check for failing hardware.

8 Tuning and Mitigation

• Reduce Process/Thread Count: If a specific application is spawning too many threads, adjust its configuration (e.g., web server worker threads).

• Adjust Scheduling: Use nice or chrt to prioritize critical processes, reducing contention:

nice -n -10 <command>

CPU Affinity: Pin processes to specific CPUs with taskset to minimize switching:

taskset -c 0-3 <command>

Kernel Tuning: Increase time slice duration by tweaking /proc/sys/kernel/sched_min_granularity_ns (requires root):

echo 10000000 > /proc/sys/kernel/sched_min_granularity_ns

This reduces preemptions but may affect latency-sensitive apps.

Upgrade Hardware: If the workload exceeds CPU capacity, more cores or faster CPUs might be needed.