Fine grained control over event queue

[Rowan Reeve]

Hi Kenton,

Our company is attempting to use Cap'n Proto in a soft-realtime system where we need tight control over the resolution of promises. Our problem is that we have some existing synchronous processing which needs to run frequently and timely (on the order of 100 microseconds) but our system must also service incoming RPCs (which might access results of said processing) without missing a single processing window.

What we would like to be able to do roughly corresponds the below pseudocode inside our server:

loop forever:

  if before synchronous processing window:

    poll single event

  else:

    do synchronous processing

WaitScope::poll almost fulfills our needs, but it causes all new events since the last poll to be picked up and their corresponding promises to be resolved, in this case to incoming requests, which can occasionally cause us to miss our window. Our individual RPC functions are relatively short-lived, but when receiving many in a short span of time, we encounter this timing issue.

I think what we're looking for is an overload to WaitScope::poll limiting the turn count:

https://pastebin.com/T4eFM60F

Your thoughts? Otherwise could this be something that might be accepted as a small pull request?

Useful details:

• C++ 17 with official capnp/kj library
  
• Linux 5.x kernel (so UnixEventPort as async provider)

Rowan Reeve

[Kenton Varda]

Hi Rowan,

Hmm, this feels to me like more of a use case for threads or signal handlers. Trying to limit processing time by limiting turn count seems precarious to me as the amount of time spent on any turn could vary wildly.

That said the change you propose seems not too bad and I'd accept it, I just worry it's the tip of the iceberg here. Like will you find out that some particular callbacks in the RPC system can be especially expensive, and need to be counted as multiple turns? I think that would be going to far.

If by any chance your synchronous processing could be made safe to run in a signal handler (requires using lockless data structures), I think a timer signal could be a pretty neat solution here.

-Kenton

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[Rowan Reeve]

Hi again Kenton,

Thank you for your valuable input. Yes, agreed that it's a bit precarious but it'd be a useful stop-gap that is currently tested and working for us, albeit with a much rougher self-maintained patch.

For more context about our situation: we're constrained by a large existing software stack running on an embedded system that has been extended to use capnp for some IPC but still assumes a single real-time thread. The system configuration is such that that the main thread runs on its own dedicated physical core to meet some bandwidth requirements for I/O over Ethernet, and introducing another loop or thread could negatively impact that (locks on shared buffers or message passing). We'd need quite some time to investigate, test and benchmark any changes in that direction.

I will be raising a PR on GitHub later today with the first solution (optionally limiting event loop turns on poll) similar to the previously linked pastebin and you can see how you feel about it whenever you find time to review.

Thanks,

Rowan Reeve