This is a framework for co-operative allocation of domains for multicore OCaml.
The overall goal is to make it possible for asynchronous and parallel programming libraries to co-exist and co-operate profitably with minimal loss of performance so that applications do not have to unnecessarily choose between competing library ecosystems.
The basic problem with domains in OCaml is that they are very expensive. They are relatively expensive to create and, more importantly, performance drops when there are more domains than available hardware threads even when domains are merely idle. Even if the cost of idle domains might be reduced via future runtime optimizations, having at most one domain per available hardware thread can provide performance benefits.
This basically means that libraries that want to co-exist and co-operate should never create domains on their own.
On the other hand, what (low-level) asynchronous and parallel programming libraries want to do is to schedule operations on one or more domains. A scheduler typically runs a loop on one or more domains that takes operations from some sort of dispenser (queue, stack, work-stealing deque, ...). The exact details of how a scheduler is implemented tend to be very important.
The approach of idle-domains is to manage a number of domains and keep track
of when a domain is idle. Co-operative libraries can then attempt to allocate
idle domains for running their schedulers.
idle-domains is not intended to be a general purpose scheduler and does
not make use of any unbounded dispensers for handling requests. idle-domains
merely manages the allocation of a finite number of domains. The goal is to
provide a layer on top of which effective and efficient schedulers can be
written.
See the reference manual or the Idle_domains.mli signature for the API.