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Comment by rwmj | Hacker News Reader

I've never thought that preemptive multitasking is "such a giant fragile pain in the a". I think it's pretty useful. What has been the problem with it?

We could probably do with partitioning the desktop though, but that's easily done with a bit more cgroups.

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The biggest problem is that context switching overhead becomes the main "work" being done, rather than the actual work of the threads, for some core count n (which depends on workload).

You can already saturate an entire core just copying data from a NIC. Of course, enter BPF and all that jazz. Plus it doesn't play as nice with BIG.little multiprocessing (AMP - asymmetric multiprocessing).

edit: I should note I am not advocating getting rid of preemption entirely, because it is needed for at least one processor, but having a fixed quantum causing a context switch on application CPUs is highly disruptive, both from the operation itself, and from blowing the cache all the time. Incidentally, one of the main reason for green threads.

One modern systems, preemptive multitasking has a surprisingly high overhead. Much of your CPU time can be spent on context switching and the associated cache thrashing, which isn't productive. Furthermore, many kinds of macro-optimizations based on temporal locality stop working because preemption can randomly inject badly chosen computation into your carefully crafted computational sequencing. Preemptive multitasking makes it more difficult to ensure robustness under load generally because the default behavior of the system is that it will happily do the wrong thing at the wrong time under stress, because the OS has no idea what the right thing is, which can aggravate the situation. Obviously, none of this applies to systems that are typically under light-to-modest load.

For servers with hardware capable of very high bandwidth I/O, preemptive multitasking is a major drag on performance and scalability. Hence why "thread-per-core" software architectures are idiomatic for high-performance data infrastructure software these days. You don't just gain significant absolute performance, the behavior can also degrade more gracefully under extreme load.

Like with all things, it depends on what you are doing. I've designed database engines both ways and Linux is not opinionated about such things.

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