Add an allocator and a finalizer used to run the given fn with a pre-allocated resource

[dinosaure]

This is a draft about #22 which asks to allocate some resources, pass them to the given fn and free them then. They should not mitigate results but I need a double-check from you @craigfe 👍 . Currently, fact.html did not change so much with this PR. I believe that it's fine.

[dinosaure]

Currently, I did not implemented make_indexed_with_resource. Tell me if you need that.

[craigfe]

Thanks a lot for this excellent feature. I've ported ocaml-uring to use it here, and this API seems very natural to me.

Two thoughts:

• I did indeed need make_indexed_with_resource, which I quickly did in this commit. Perhaps the only interesting point is that I needed to pass the int index argument to allocate in order to create a uring of the correct size.

• The uring resource is very heavy-weight (& individual bench runs are quite fast), and so allocating an array of them for each iteration inside the time quota quickly crashed the benchmark with ENOMEM. I changed the implementation to instead share a single resource over a run in this commit, and this works for the ocaml-uring usecase. I imagine it's still useful to have distinct resources in some cases, so I'm not sure what to recommend here. (Do we want both behaviours? Is there an API to unify them?)

[dinosaure]

The uring resource is very heavy-weight (& individual bench runs are quite fast), and so allocating an array of them for each iteration inside the time quota quickly crashed the benchmark with ENOMEM. I changed the implementation to instead share a single resource over a run in this commit, and this works for the ocaml-uring usecase. I imagine it's still useful to have distinct resources in some cases, so I'm not sure what to recommend here. (Do we want both behaviours? Is there an API to unify them?)

Yeah I think your case is legitim and the initial case (one resource per run) is legitim too. I will try to merge your commit and prepare something on the API level. Thanks for your feedback, it helps a lot.

[dinosaure]

/cc @craigfe I finalize the API which gives you the choice between the allocation of a single resource or multiple resources. I need to check a bit the assembly generated and if OCaml is able to inline correctly <.> (but I think it can not) to be sure that we don't do some jump/extra allocation/etc.

Can you test this new version with your usecase and see if something is wrong about results? Better, can you give me the link to the repository to reproduce on my side?

[dinosaure]

I did a deep look on the assembly generated by the compiler and it seems that if we use multiple resources for each run, we can trigger the GC via the unsafe_array_get operation. That mostly means that it's not so bad and in your context, you should not have noises around the execution of your function - however, we should precise that "a resource per run" can lead some noises at some points.

[dinosaure]

Finally, if you agree with this interface, I can merge it and prepare a release then 👍.

[craigfe]

Thanks for this improvement! LGTM.