Master Linux Scheduling: Real‑Time, Normal, Batch & Idle Policies Explained

Linux provides several scheduling policies for both real‑time and non‑real‑time processes. The real‑time policies include SCHED_RR (round‑robin, each process gets a maximum CPU time slice) and SCHED_FIFO (first‑in‑first‑out, a process runs until it blocks, yields, or is pre‑empted by a higher‑priority task).

SCHED_NORMAL is the default policy for regular processes, implementing a standard round‑robin algorithm. SCHED_BATCH is tuned for batch jobs such as data compression; it avoids pre‑emption to improve cache usage. SCHED_IDLE is for very low‑priority tasks (nice value less than 19).

Example commands to inspect the system: lscpu Sample output shows a dual‑CPU x86_64 system with two sockets, each with one core.

Testing the policies with chrt:

# chrt -f 10 top
# chrt -f 1 md5sum /dev/zero &
# chrt -r 1 sha1sum /dev/zero

Observations: the md5sum process run with SCHED_FIFO (priority 1) consistently occupied the CPU ahead of top. When using the round‑robin policy ( SCHED_RR) for sha1sum, the process was not visible in top, indicating it was not scheduled as aggressively.

These experiments demonstrate how different scheduling classes affect CPU allocation and process visibility.