Re: [capnproto] Fine grained control over event queue

['Kenton Varda' via Cap'n Proto]

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

On Wed, Jun 22, 2022 at 2:17 PM Rowan Reeve  wrote:

> 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
>
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> 
> .
>

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[capnproto] 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

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