Operating Systemsmedium

What is context switching and why is it expensive?

Tags

#context-switch#scheduler#performance

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Answer

Context switching is when the CPU switches from one thread/process to another. It’s expensive because the OS must save/restore registers, switch memory mappings, and flush caches/TLBs.

Advanced answer

Deep dive

Switching work adds overhead and disrupts locality:

Save/restore registers and stack pointers.
TLB and cache effects reduce CPU efficiency.
Scheduler decisions add latency under load.
Frequent switches hurt throughput for CPU-bound tasks.

Examples

High thread count scenario:

1000 runnable threads -> frequent switches -> lower cache hit rate -> slower throughput

Common pitfalls

Creating too many threads for CPU-bound work.
Using blocking I/O without thread pools.
Confusing context switches with process forks (different costs).

Interview follow-ups

How do you measure context switch overhead?
When do you prefer async I/O over threads?
How do caches and TLBs affect performance?

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