Cooperative User Threads vs. Preemptive Kernel Threads

James Robertson, Cooperative Threading:

> Well, in Cincom Smalltalk, this model gives you predictability –
> you know exactly what a thread is going to do. The issue with runaway
> threads rarely comes up for a simple reason – most processes end up
> pausing for I/O (user input, db access, file access, sockets –
> what have you). That wait for I/O state is what prevents a problem
> from arising.

This is a classic problem and I’m honestly surprised to find out that Cincom Smalltalk implements cooperative user-level threads rather than supporting preemptive kernel threads.

Here’s what I posted in response to James, unattributed thanks to the torturous comment interface on his blog:

> One issue with cooperative threads relative to preemptive
> OS-supplied threads is that you get far less opportunity
> for true concurrency within an application. In an era when
> multi-core processors are becoming significantly more common,
> this is becoming exceptionally important to application
> developers. It’s not just about doing I/O concurrently with
> other operations or allowing an application to perform
> multiple tasks at once; it’s about allowing a task to be
> completed faster because more efficient use is being made
> of machine resources. This is why I take an extremely
> skeptical view of user-level threading packages, especially
> in software built on platforms that have reasonable
> kernel-level threading.

You’ll note that the various threading APIs in Mac OS X are all built on kernel threads.

Furthermore, the Mach microkernel schedules exclusively in terms of threads. The microkernel doesn’t even have a conception of processes! It only knows about collections of resources — *tasks* — such as address spaces and IPC ports, and flows of control — *threads* — that it can schedule on processors.

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