Implement asynchronous scheduler

this adapts the scheduler implementation to the IQueueingEventProcessor interface, which can asynchronously schedule events, a requirement stated by @frankm to be able to run it online.
For details about the interface, see gaudi/Gaudi!966 (merged).
The performance doesn't suffer, if anything it gets better (@clemenci made the tests there)
TLDR: much benefit, no cost.

changed title from WIP start implementing asyncexecuteevent to WIP asynchronous scheduler

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I've been thinking about the end-of-data case, and I can only imagine it handled in the createEventContext, which can return an invalid context in that case.

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• 810f2ddb - continue

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@clemenci Using futures and promises makes the main thread ~25% slower.. In a job where the main thread is the throttling one, i go from 250k evts/s to 200k evts/s.
Maybe we want to support two versions?
I tested that this slowdown is not caused by any other changes by just appending two dummy lines into the current version of the scheduler at the start of executeEvent

std::promise<std::tuple<StatusCode, EventContext>> promise;
auto future = promise.get_future(); 

I'm not surprised there's overhead, but I'm surprised it's that big.

I'd like to have a look next week. (may be batching might help?)

For sure I do not want to support 2 versions of anything. My question is more if we want to keep a communication channel open between the main thread and the tasks, and, if yes, which is the fastest option.

During the hackathon we discussed the possibility of using queues instead of promise+future pairs.

I'd like to prototype a change to the interface in that direction (probably not backward compatible).

closed

mentioned in merge request gaudi/Gaudi!966 (merged)

reopened

I updated the changes to use IQueuingEventProcessor instead of IAsyncEventProcessor (see gaudi/Gaudi!966 (merged)).

The unavoidable overhead becomes negligible with the queue approach, see LBCOMP-21.

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• 3db2f8c0...c7364443 - 325 commits from branch master
• 35211eb2 - Merge branch 'master' into NN_asyncscheduler

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• 9ce492dd - Merge branch 'master' into NN_asyncscheduler

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I also merged master to fix the conflicts. @nnolte, please check that the changes make sense to you and I didn't screw up anything.

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• 44ddcf44 - Apply suggestion to Hlt/HLTScheduler/src/HLTControlFlowMgr.cpp

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shall we merge this?

unmarked as a Work In Progress