Le jeudi 10 novembre 2011 à 16:25 +0100, Dave Taht a écrit : > Two notes: > > 1) Getting 'time' from the kernel is expensive. And: System time wanders. > > mac80211 Wifi devices however do export a get_tsf function which could > be used as a relative-to-the-queue clock - and we actually don't need > accuracy down to the level of get_tsf (25ns) > getting 'time' from kernel is not that expensive, depending on resolution you need. We are not going to use timestamps from devices ! If ms resolution is enough (say you want to drop packets if they stay more than 100ms in qdisc), jiffies is a single memory read. If needing us or ns resolution, psched_get_time() uses ktime_get(), and is used in CBQ, HTB, HFSC, TBF, so if it was expensive we would have big problem right now :) 2) You need to get a timestamp on entry to the first queue and check > against the allowable latency on exit from the last. So to construct a > tc chain you'd want a tfifo (timestamp on entry), fifot (check > timestamp against limit on dequeue), and for the simplest of > applications : tfifot (timestamp on entry, check on exit) That would be not very practical. I would see a new Qdisc/Class property, like the rate estimator, that we can attach to any Qdisc/Class with a new tc option. Even without any limit enforcing (might be Random Early Detection by the way), it could be used to get a Queue Delay estimation, using EWMA avqdelay = avqdelay*(1-W) + qdelay*W; W = 2^(-ewma_log); tc [ qdisc | class] add [...] [est 1sec 8sec] [delayest ewma_log ] .. tc -s -d qdisc ... qdisc htb 1: root refcnt 2 r2q 10 default 1 direct_packets_stat 0 ver 3.17 Sent 3596219 bytes 2567 pkt (dropped 238, overlimits 3797 requeues 0) rate 2557Kbit 215pps backlog 0b 0p requeues 0 delay 91ms tc [ qdisc | class] add [...] [est 1sec 8sec] [delaylimit max ] .. tc -s -d qdisc ... qdisc htb 1: root refcnt 2 r2q 10 default 1 direct_packets_stat 0 ver 3.17 Sent 3596219 bytes 2567 pkt (dropped 238, overlimits 3797 requeues 0) rate 2557Kbit 215pps backlog 0b 0p requeues 0 delay 91ms delaylimit 100ms (dropped 12) -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
