Linux is a cross-architecture kernel, often packaged in neat distributions. These distributions contain GNU software, which distributes the C standard library and common utilities.
soft real-time
Architecture
uses a FCFS and RR scheduler SCHED_FIFO, SCHED_RR
priority range
soft real-time 99 to 0 -100 to -1
normal niceness -20 to 19 0 to 39
priority -100 == real-time RT
Scheduling
Linux has a broad history of using different scheduling algorithms:
- In Linux 2.4, it used a single scheduler for multiprocessor CPUs.
- In Linux 2.6, it used a compromise scheduler between a global and per-CPU scheduler, with an complexity. The scheduler was able to re-balance per-CPU queues (taking a process from another CPU and assigning it to idle CPUs). It relied on “processor affinity” if a process wanted to be scheduled on the same core.
- It currently relies on a completely fair scheduler, which permits good interactivity for foreground processes.
- Linux also implements first-come-first-serve (FCFS) and round-robin (RR) scheduling, in
SCHED_FIFOandSCHED_RR, respectively.
Soft real-time processes are always scheduled with the highest priority processes first. Normal processes have their priorities adjusted based on aging. Linux maintains a priority range (a Linux priority), between -100 and 39.
- Soft real-time processes have a priority between 99 and 0. Their Linux priority is between -100 and -1.
- Normal processes have a “niceness” between -20 and 19. Their Linux priority is between 0 and 39. The order these are described in matters.
