Document how to use Domainslib from Eio #489
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I've expanded the example a bit to show how to start the main loop, due to some confusion in https://discuss.ocaml.org/t/interaction-between-eio-and-domainslib-unhandled-exceptions/11971. |
| ```ocaml | ||
| let pool = Domainslib.Task.setup_pool ~num_domains:2 () | ||
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| let fib n = ... (* Some Domainslib function *) |
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Do you think it makes sense to stress that you cannot perform a call that may perform an (Eio) effect in the Domainslib async task because that may be run in a different domain that no longer has the effect handler installed?
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Good idea. I've added a bit of text saying you can mostly only use functions from the appropriate library in each domain.
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| let run_in_pool fn x = | ||
| let result, set_result = Promise.create () in | ||
| let _ : unit Domainslib.Task.promise = Domainslib.Task.async pool (fun () -> |
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I initially assumed that the send that happens internally with Task.async would be safe to call from non-domainslib domains, but as I'm now looking into adding POC domain local await support to domainslib, I realize that this works somewhat accidentally.
When you create the pool
let pool = Domainslib.Task.setup_pool ~num_domains:2 ()the requsted n=2 domains are started. Additionally an internal "multi channel" is created with n+1 capacity. The +1 reserves an extra internal "channel" for the domain that calls Task.run.
When Task.async is called, it looks up the internal channel for the current domain. If no channel has already been allocated for the current domain, it then allocates a channel for the domain from the reserved n+1. This works as long as Task.async is only called from a single additional domain. If you had two (or more) Eio domains, then only one of those could call Task.async.
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Interesting. I guess Task.run has the same limitation, then? Would be worth documenting that (in Domainslib and here).
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Yes, I think it has the same limitation.
At any rate, I'm looking into how to potententially change that to be more domain safe.
/cc @kayceesrk