[Bro-Dev] Improving Bro's main loop

[Siwek, Jon]

> On Feb 9, 2017, at 12:02 PM, Robin Sommer <robin at icir.org> wrote:
> 
>    - We need to maintain some predictability in scheduling, in
>      particular with regarding to timing/timers. Bro's network time
>      time is, by definition, defined through I/O. My gut feeling is
>      that we need to keep the tight coupling there, as otherwise
>      semantics would change quite a bit.
> 
>    - Related, another reason for time playing such an important role
>      in the I/O loop is that Bro needs to process its soonest input
>      first. That's most important for packet sources: if we have
>      packets coming from multiple packet sources, earlier timestamps
>      must be processed before later ones across all of them. 
> 
>    - Time is generally complex, we have three different notions of
>      network time actually, all with some different specifics: time
>      during real-time processing, time during offline trace
>      processing, and pseudo-realtime.

Also not sure to what degree coupling related to time/timers can be reduced, though I think at least an initial refactor of the run loop could be done such that it doesn’t change much related to how time currently works.  Then maybe later or during the refactor, it will get easier to see what exactly can be improved.

>    - I believe we need to maintain the ability to have I/O loops that
>      don't have FDs.

Yep, don’t think there will be a problem there.

>    - I like the idea of using CAF, including because it's going to be
>      a required dependency anyways in the future. I would also like
>      it conceptually to move I/O to actors, and I'm wondering if even
>      packets sources could go there. However, I can't quite tell if
>      that's feasible given other constrains and how other parts of
>      the system are layed out (including that in the end, everything
>      needs to go back into the main thread before being further
>      processed; at least for the time being).

I do think even packet sources could get moved into actors.  My initial idea for the main loop refactor is for it to be a single actor waiting for “ready for processing” messages from IOSources, and then for each IOSource to be responsible for its own FD polling (if it needs it).  That way, the main loop doesn’t care about FDs at all anymore and if an IOSource needs to poll FDs it can just use poll() in its own actor/thread for now (my guess is that most IOSources will just have a single FD to poll anyway or that the polling mechanism isn’t a very significant chunk of time for ones that may have more, but the only way to answer that is to actually do the performance testing.)

>    - One of the trickiest parts in the past has been ensuring good
>      performance on a variety of platforms and OS versions. Whatever
>      we do, it'll be important to do quite a bit of test-driving and
>      benchmarking. Let's try to structure the work so that we can get
>      to a prototype quickly that allows for some initial performance
>      validation of the approach taken.

Sure.  I was also expecting to try and just get something working without any significant overhauling of any of Bro’s systems.

- Jon