Queues: generalize PIFOs

[anshumanmohan]

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At present our PIFOs can only handle:

• Two flows.
• A policy that attempts to give those two flows equal shares.

These limitations of PIFOs also limit PIFO trees.

There is room for generalization on a few axes. I see these as orthogonal, though of course they'd be more powerful if combined. I think these are doable even just using our approach where a PIFO merely orchestrates some n FIFOs, where n can be determined in advance.

• [ ] A PIFO that handles n flows and affects the policy du jour on them.
• [ ] A PIFO that allows strict prioritization among its children: strictly prefer flow A over flow B over flow C, and so on. This PIFO would still be work-conserving: if flow A were silent and flow B had items, we'd send B's items until A became chatty again, and we'd send C's items only if A and B were both silent. This generalizes easily to n flows.

[sampsyo]

This all seems quite sensible! Except that I don't quite understand one thing: what does "strict prioritization" mean? Is it an alternative to fairness?

[anshumanmohan]

Ah, sorry! I'll edit the text above to explain, but yes, this is a different scheduling policy that I think is easily within our grasp using just our simple approach. The idea is to strictly prefer flow A over flow B over flow C, and so on.

[anshumanmohan]

Just documenting a wrinkle that has come up now that @csziklai has started working on this! Let's just focus on round-robin scheduling with 3 flows.

Review: n=2

As you know, we have a little mechanism to record which of the sub-queues of the PIFO is hot, i.e., ready to be popped/peeked next. When possible we pop/peek from that queue and flip the pointer hot, and otherwise we (begrudgingly) pop/peek from the other queue and do not flip hot.

A Glimpse of the Challenge: n=3

As documented in the glossary, round-robin behaves as expected when there are three flows that are all sending packets. However, if one other flows falls silent, the expectation is a little subtle. rr(A, B, C), where A falls silent, needs to behave like rr(B, C).

In implementation-land, say we have a PIFO with three FIFOs as sub-queues. Call them A, B, and C. Say that hot points to A but A has no packets, while B and C do have packets. Now pop is called repeatedly on the PIFO. Which sub-queue do we pop begrudgingly? We can't just always pop B... that would create either strict(B, C) or fifo(B), depending on how cleverly we do it. Anyway, either is wrong.

To get rr(B, C) behavior out of rr(A, B, C) with silent A, we actually need to again swap between B and C! This may be okay if n were hardcoded to 3 – we might have a secondary hot-pointer that toggles between the two non-hot choices – but we need to think a little more carefully about truly general n!

[anshumanmohan]

Here's a sketch proposal. Warning that it's a quick and dirty sketch mostly focussed on n=3, but I'd like to surface it so we can pick holes in it later today.

If we have three flows A, B, and C, we need to consider the following eight cases in terms of their chattiness.

Flows	Case 1	Case 2	Case 3	Case 4	Case 5	Case 6	Case 7	Case 8
A	chatty	silent	chatty	chatty	silent	silent	chatty	silent
B	chatty	chatty	silent	chatty	silent	chatty	silent	silent
C	chatty	chaty	chatty	silent	chatty	silent	silent	silent

Whatever our policy, it needs to behave correctly in all eight cases. Cases 1 and 8 are sorta easy, and cases 5, 6, and 7 are not so bad because there is only one correct flow to send from. Cases 2, 3, and 4 are harder because, see above, we need to alternate between the two flows that are chatty.

I have sketched up the following, and I can walk through the thinking IRL. I have abstracted away "transmit ans", which is how we'd tell the world that we found an answer.

hot = 0 # the point is just that hot has been set for us arbitrarily.
# the value 0 does not matter.

ans = pop(hot) 
if ans: # the target queue had something, great! 
  hot++ # move the target. incr is assumed to overflow at n, which is 3.
  explore = 0 # set/reset exploration probe.
  transmit ans
else:
  repeat(3): # we limit our explorations to n steps.
    ans = pop(hot + explore) # this addition is assumed to overflow at n, here 3.
    # So if hot = 2 and explore = 1, this will try to pop queue 0.
    explore++
    if ans:
      transmit ans
      break # break out of this repeat loop
  # the n-step exploration has ended, because of success or failure.
  if not ans:
    underflow error

[csziklai]

Here is the updated pop() code, as discussed in the meeting.

   def pop(self) -> Optional[int]:
       """Pops the PIFO."""
       print("pop " + str(self.hot))
       if self.pifo_len == 0:
           if self.error_mode:
               raise QueueError("Cannot pop from empty PIFO.")
           return None
       self.pifo_len -= 1
       original_hot = self.hot

       while True:
            val = self.data[self.hot].pop()
            if val is not None:
                self.increment_hot() 
                return val
            else:
                self.increment_hot()
                if self.hot == original_hot:
                    return None```

[anshumanmohan]

Thanks Cassandra! So yes, to sum: the problem I chalked out was reasonable, but my proposed code was actually still a little off. The code above takes a slightly different tack to achieving round-robin. It is simpler and will translate to Calyx more easily.

In English, the policy is simply:

• Start with some hot pointer.
• In an infinite loop:
  • Pop hot.
    • If it succeeds, great. Increment hot and return the result you got.
    • If it fails, increment hot. This is an attempt to keep looking for a value in some other queue. The queue that was just recently hot has missed its turn, and we'll just get to it later.

Two notes:

• These are all wraparound increments, i.e. add modulo n where n is the number of flows managed by the PIFO.
• We have a little original_hot variable, that we check to ensure that we haven't looped all the way around. This may in fact be unnecessary, since we know from line 4 that the PIFO is nonempty. Anyway let's just keep it as a redundancy, watch everything succeed, and then maybe we can remove that variable and the last two lines of the code snippet.