Why Full Preemption?

(Slight Return)

Paul Tarvydas

Apr 03, 2025

Q1: Why are multitasking libraries and operating systems difficult to use and bloated?

A: Because they use full preemption.

Q2: Why do multitasking libraries and operating systems use full preemption?

A: That is how multiple threads can be simulated on a single processor.

Q3: Why do we simulate multiple threads on a single processor?

A: Processors are expensive, so we can only afford to use one processor.

Q4: Why are processors expensive?

A: Processors used to be expensive, but they are inexpensive now.

Q5: Why don't we use multiple processors, one for each thread?

A: Because that is the way we always did it, starting in the 1960s, when processors were expensive.

Conclusion

Use multiple processors. We no longer need to simulate multiple threads on a single processor. If we must simulate multiple threads on a single processor, we must not simulate scenarios, like memory sharing, that are not a natural way to use multiple processors. That just invites accidental complexity.

Acknowledgement

5 Whys approach suggested by Daniel Pink in Masterclass.

[This article was previously published on my old blog].

Programming Simplicity

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