[BB]: So that begs just one remaining question: Q: Do you have tests showing any benefit, specifically comparing with and without this "< QDELAY_REF/2" heuristic?
If we set the QDELAY_REF too low, we have seen losing throughput. This is related to your question regarding qdelay_old_. You are using sojourn time while we kept the state in qdelay_old_ which can become stale if update_interval is long. Rong On 3/28/17, 6:41 AM, "aqm on behalf of Bob Briscoe" <[email protected] on behalf of [email protected]> wrote: >Rong, > >Some comments inline. And one remaining question at the end... > >On 28/03/17 02:04, Rong Pan (ropan) wrote: >> Bob, >> >> Sorry for the late reply. I have been traveling. Please see inlineŠ >> >> Rong >> >> On 3/23/17, 5:01 PM, "aqm on behalf of Bob Briscoe" >><[email protected] >> on behalf of [email protected]> wrote: >> >>> Rong, Preethi, Greg, Fred, and others involved in PIE, >>> >>> You may recall that when we wrote PI2 we didn't include any of PIE's >>> heuristics. Mostly because PI2 solved the issues they addressed >>> intrinsically. But we left some until we had checked their benefit, >>> which is what I'm doing now... >>> >>> My first question is about this heuristic in PIE: >>> >>> //Safeguard PIE to be work conserving >>> if ( (PIE->qdelay_old_ < QDELAY_REF/2 && PIE->drop_prob_ < >>>0.2) >>> || (queue_.byte_length() <= 2 * MEAN_PKTSIZE) ) { >>> return ENQUE; >>> } >>> >>> If it tests true, this block doesn't stop the calculation of drop_prob_ >>> evolving, but it disables it being able to lead to any random packet >>>drop. >>> >>> I can understand why you want to disable packet drop when the queue is >>> no more than 2 packets. >>> My question is about the first half of the logical OR. The drop_prob_ < >>> 20% test will be true under normal non-overloaded conditions. So I have >>> just realized that the qdelay_old_ < QDELAY_REF/2 test will turn off >>> random drop very often. I would expect this to radically impact the >>> behaviour of PIE. It seems to be overriding the PI controller as if you >>> are thinking "actually we don't really trust the PI controller to leave >>> it to do its thing, so we've overridden it a lot of the time." For >>> instance, whenever a single long-running TCP flow with RTT about the >>> same as the target delay is saw-toothing, this test will disable random >>> drop completely during the lower half of every saw-tooth in the queue. >>> Maybe that's OK, but... >>> >>> Without this test, the PI controller should reduce drop probability as >>> the queue sawtooths down anyway. If another flow causes the queue to >>> rise rapidly while it is under half the target, the PI controller is >>> designed to detect such an increase and translate it into drop. But >>>this >>> heuristic suppresses any drop until the queue has exceeded half the >>> target. >>> >>> So my questions are: >>> >>> Q1. What were the reasons for introducing such a frequent suppression >>>of >>> the PI algorithm (the RFC just says what this code does, not why)? >> >> To be work conserving and avoid any unnecessary drops are the main >>reasons >> behind it. >> Cisco had a not so successful algorithm before that is not work >> conserving. So we are >> extra cautious about being work conserving... >[BB] There is only a work-conservation problem if drop_early() is >applied at enqueue. That's because, at enqueue, you don't yet know >whether another packet will arrive to take the place of the packet you >are deciding to drop. > >We're shifting drop_early() to dequeue {Note 1}. So to be >work-conserving we can rely solely on the test on the other side of the >logical OR above that suppresses any drop if "backlog < 2 MTU". That's >the only heuristic that we are keeping so far, although I'm undecided >about the "< QDELAY_REF/2" test, which (as you say) might be beneficial >for other reasons than work conservation. But we have no tests that show >that yet. > >{Note 1}: Because we're using sojourn time to measure the queue, so if >we were still dropping on enqueue, each congestion signal would be >delayed twice by the queue. > >> >> >>> Q2. Why use qdelay_old_ in the test? This seems to drive suppression of >>> drop using stale state. >> qdelay_old_ is the latency state currently stored. This is for >> implementation >> Considerations as we don¹t want to calculate qdelay_ on per packet >>basis. >[BB] Understood. >We're using sojourn time per packet for the shifted FIFO scheduler >anyway, so no extra cost. >> >>> Q3. Having said that it looks like this heuristic will significantly >>> alter PIE's behaviour, in tests under a very wide range of traffic >>> conditions, link rates, mixed RTTs, traffic models etc, we have found >>> that removing the heuristics makes no measurable difference to PIE's >>> performance. So if you added this heuristic for a specific scenario, >>> please describe it, so we can test for it. >> Again, to be work conserving and avoid drops are our goal. I don¹t >> think it would be hurtful to add those safeguards. > >[BB]: So that begs just one remaining question: >Q: Do you have tests showing any benefit, specifically comparing with >and without this "< QDELAY_REF/2" heuristic? > >Given the point of a (non-ECN) AQM is to introduce the right level of >random drops, it seems strange to suppress some of them with an >additional arbitrary rule. > > >Thanks for your replies so far tho. They have helped me realize more >reasons why PIE needs these heuristics, but PI2 might not. > > >Bob > >> >> >>> >>> Cheers >>> >>> >>> Bob >>> >>> >>> >>> >>> -- >>> ________________________________________________________________ >>> Bob Briscoehttp://bobbriscoe.net/ >>> >>> _______________________________________________ >>> aqm mailing list >>> [email protected] >>> https://www.ietf.org/mailman/listinfo/aqm >> _______________________________________________ >> aqm mailing list >> [email protected] >> https://www.ietf.org/mailman/listinfo/aqm > >-- >________________________________________________________________ >Bob Briscoe http://bobbriscoe.net/ > >_______________________________________________ >aqm mailing list >[email protected] >https://www.ietf.org/mailman/listinfo/aqm _______________________________________________ aqm mailing list [email protected] https://www.ietf.org/mailman/listinfo/aqm
