Process Priority

# Check nice value of a process
ps -eo pid,ni,comm --sort=-ni | head -10
#   PID  NI COMMAND
# 12345  -5 sshd
# 12346  19 bash
# 12347   0 nginx

# Check nice value with ps
ps -p 12345 -o pid,ni,comm
#   PID  NI COMMAND
# 12345  -5 sshd

# Check scheduling priority
ps -eo pid,cls,rtprio,ni,comm | head -10
#   PID CLS RTPRIO  NI COMMAND
# 12345  TS      -   0 sshd
# 12346  RR     50  -5 nginx
# 12347  TS      -  19 batch_job

# Start a process with lower priority
nice -n 10 ./heavy_computation.sh
# Runs with nice value 10 (lower priority)

# Start with higher priority (requires root)
sudo nice -n -5 ./important_task.sh

# Change priority of running process
renice -n 10 -p 12345
# Set PID 12345 to nice value 10

# Change priority to highest (requires root)
sudo renice -n -20 -p 12345

# Change priority of all processes in a group
sudo renice -n 5 -g mygroup

# Change priority by name
renice -n 10 -p $(pgrep nginx)

# Check the result
ps -p 12345 -o pid,ni,comm
#   PID  NI COMMAND
# 12345  10 nginx

# Check I/O priority
ionice -p 12345
# best-effort: pid 12345, class 2, prio 4

# Classes:
# 0 = none (use best-effort)
# 1 = real-time (highest I/O priority)
# 2 = best-effort (normal scheduling)
# 3 = idle (only when no other I/O)

# Set I/O priority
ionice -c 3 ./backup.sh      # Idle I/O priority
ionice -c 2 -n 0 ./critical  # Best-effort, highest priority
ionice -c 1 ./emergency       # Real-time I/O priority

# Check I/O priority of all processes
ionice -p $(pgrep -f ".sh") 2>/dev/null | head -10

# Set real-time priority (requires root)
sudo chrt -f 50 ./realtime_task.sh
# FIFO, priority 50

# Set batch scheduling
sudo chrt -b 0 ./batch_job.sh

# Check scheduling policy
chrt -p 12345
# pid 12345's current scheduling policy: SCHED_OTHER
# pid 12345's current scheduling priority: 0

# List all real-time processes
ps -eo pid,cls,rtprio,comm | grep -E "FF|RR"
# 12345  FF     50  nginx
# 12346  RR     50  nginx

# Check for high-priority processes blocked on locks
sudo perf record -g -a sleep 5
sudo perf report

# Look for futex calls in stack traces
sudo bpftrace -e 'kprobe:futex_wait_queue { @[comm, pid] = count(); }'

# Check for processes stuck in D state (disk sleep)
ps -eo pid,stat,ni,comm | grep " D "
# High-priority processes stuck in D state may indicate inversion

# Priority inheritance (PI) mutexes
# Linux kernel uses PI futexes to prevent inversion
# Check with:
cat /proc/sys/kernel/sched_rt_runtime_us
# 950000 (95% of CPU for RT processes)

# Lower priority for batch jobs
nice -n 10 tar czf backup.tar.gz /data

# Lower I/O priority for backups
ionice -c 3 -n 7 rsync -av /data/ /backup/

# Higher priority for interactive services
sudo renice -n -5 -p $(pgrep sshd)

# Higher I/O priority for databases
sudo ionice -c 1 -p $(pgrep postgres)

# Background tasks at lowest priority
nice -n 19 ionice -c 3 ./maintenance.sh &

# Check if tuning worked
ps -eo pid,ni,cls,rtprio,comm | grep nginx