It has been several years since I glanced at Barrelfish. I am plowing into this document. These are good rules. There are different good rules. It is a good sign that they begin here. Keykos said early on: the kernel runs privileged, unmapped, without interrupts, even even without page faults. All other code runs mapped, interrupts enabled and with demand paging.

Contrast with Keykos

I like their style of description and I will thus contrast Barrelfish with Keykos.


MP is more important today than when Keykos was developed. We had detailed plans for MP but no code. We planned shared RAM holding shared mutable data and a system of classic shared and exclusive data locks.

The Barrelfish ‘CPU driver’ is much like the code that defines the behavior of the Keykos domain kernel object.

“If there are no such handlers to execute, the CPU driver executes a user-space task.” This suggests a queue of messages to the kernel. This is one convenient mechanism of inter-core signals. Keykos has not settled on a solution to this but another Keykos kernel rule is that all storage (including that for message queues) be ascribed to a user account and instantly recoverable (like space in a cache). I await hearing about the discipline to avoid queue overflow. Keykos and Barrelfish kernels are alike in that there is no kernel stack state except during one of the kernel events.

It isn’t clear which the domain is: the dispatcher or application. The Keykos domain can run on any available CPU in a symmetric system. Some hardware imposes a cost in switching in that domain specific data is physically associated with a particular core. Cache and floating vector state comes to mind. We have found that keeping it logically separated is easy, while admitting some execution cost. It sounds as if application logic is necessary for homogeneous CPUs to serve a population of users.

From the description of the relation between the kernel and the dispatcher it sounds more complex to me than the Keykos model; perhaps an illusion.

How comes it that a system task (performed by kernel code?) has any dispatchers? Is a dispatcher code? Does it run privileged? They refer to K42 in which a ‘dispatcher’ is code that has its own address space some of which is pinned.

My opinion is that classic symmetric MP with coherent memory is strongly limited by hardware considerations to no more than 8 CPUs. However even four homogeneous CPUs with a shared CPU ready queue are an important configuration to support. The dispatcher design with distributed CPU ready queues don’t seem to support this. Perhaps coherence between slower caches is strategic.

Back to Barrelfish: Keykos planned on a type of (Keykos) domain for each sort of CPU. Calls between types would necessarily be between CPUs perhaps via some sort of kernel message queue which we had hitherto avoided. Hopefully these would find their data mutually comprehensible. Heterogeneous domain types could even be supported for imaginary ISA's given a user mode CPU simulator.