Operating Systems
tags: - cocubes - technical - operating-systems
Operating Systems
Process versus thread
| Process | Thread |
|---|---|
| independent executing program | lightweight execution unit within a process |
| separate address space | shares process memory/resources |
| context switch usually heavier | context switch usually lighter |
| communication needs IPC | threads communicate through shared memory |
CPU scheduling
| Algorithm | Idea | Main issue |
|---|---|---|
| FCFS | first come, first served | convoy effect |
| SJF | shortest burst first | requires burst estimate; long jobs may starve |
| SRTF | preemptive SJF | more context switches |
| Round Robin | fixed time quantum | very small quantum means overhead; huge quantum resembles FCFS |
| Priority | highest priority first | starvation; aging reduces it |
- Turnaround time = completion time − arrival time.
- Waiting time = turnaround time − CPU burst time.
- Response time = first CPU start time − arrival time.
Synchronisation
A critical section accesses shared data. Problems include race conditions and lost updates.
- Mutex: mutual exclusion lock, normally one owner.
- Semaphore: integer synchronisation primitive; binary semaphore can act like a lock, counting semaphore tracks several resources.
- Deadlock: processes wait forever for resources.
Four necessary deadlock conditions:
- mutual exclusion;
- hold and wait;
- no preemption;
- circular wait.
Breaking one condition can prevent deadlock.
Memory management
| Term | Meaning |
|---|---|
| Paging | fixed-size pages and frames |
| Segmentation | variable-size logical sections such as code/data/stack |
| Virtual memory | uses disk to extend apparent memory |
| Page fault | requested page is not currently in main memory |
| TLB | cache for recent page-table translations |
| Internal fragmentation | unused space within allocated block |
| External fragmentation | free space split into small scattered blocks |
Demand paging loads a page when needed. A page fault is normal in demand paging; it is not "a corrupted page."
File systems and I/O
- File descriptor: integer handle used by a process to access a file/stream.
- Buffering reduces expensive I/O operations.
- Spooling queues jobs for a device, such as a printer.
- DMA lets an I/O device transfer data to memory with minimal CPU involvement.
Operating-system quick comparisons
- Kernel mode can execute privileged instructions; user mode cannot.
- An interrupt is usually hardware/external event; a system call is a controlled request from user program to OS.
- A context switch saves one execution state and loads another.
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