Working with the Unix OS

Process Scheduling
12. PROCESS SCHEDULING
The Scheduler
The kernel is responsible for sharing CPU time between competing processes.
Multi-Level Priority Queue
- linked list of runnable processes
Processes are allocated CPU time in proportion to their importance. Time is allocated in fixed size units called
"time quantums" (~ 1/10 second).
Process Table
Next PID PPID Stat
MLPQ ---> . 36 12 R . ---> Process 34
| -- free entry
0 .-- | - 18 1 S
1 .-- | free entry
2 - | -> - 12 1 R . ---> Process 12
3 - | free entry
4 - | -> - 48 1 S
----> - 1 - R . ---> Process 1
Scheduling Rules
- Every second, scheduler recalculates priority of all runnable processes - organizes them into priority
queues.
- Every 1/10 sec, the scheduler selects highest priority process in priority queue and allocates it the CPU.
- If process is runnable at end of time quantum, it is placed at end of its priority queue.
- If process sleeps on an event during time quantum, the scheduler selects next runnable process.
- If process returns from system call during time quantum, and higher priority process is ready to run, the
lower priority process is preempted.
- Every hardware clock interrupt (1/100 second), the process's clock tick count is incremented, every 4th
tick, scheduler recalculates priority.
priority = K1 / (recent CPU usage) + K2 / (nice setting)
- A process's priority diminishes if it uses a lot of CPU in a window of time.
- An interactive process waits for a user to press a key, it uses no CPU time and thus its priority level rises.
Thus interactive processes obtain good response times.
Memory Management
Sharing of RAM between processes (secure, efficient)
! Memory Pages
Allow processes bigger than RAM capacity to execute. RAM (code, data, stack) divided into fixed-size pages,
analogous to, disk divided into fixed-size blocks.
The size of memory page is set to size of disk block. Only pages of process, currently accessed or recently
accessed are stored in RAM pages, the rest are on disk.
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