Re: Scheduler Mechnisms!
From: Zhenyu Wu <hidden>
Date: 2005-01-06 02:01:35
Jonathan, Yes, I think you are right, RED can be applied to any qdisc. The "RED" you mentioned here is the RED algorithm, right? I know, in linux kernel (sch_red.c), which implements the RED algotithm, just indeed uses a FIFO queue, do you think so? So, if there are serveral RED qdiscs which attach to different classes, a scheduling mechanism is needed, right? If what i have said is right, it seems that there are no scheduler mechanisms for serveral FIFO qdiscs(or RED qdiscs) in linux kernel. Of course, as we can see, CBQ which can be regard as a scheduling queue also has a scheduler mechanisms WRR. But if the FIFO is attached to the CBQ classes, then, when CBQ schedule different queues(FIFOs) using WRR, will the packets in different queues are scheduled in turn? In another condition, if i add a scheduling mechanism such as WRR behind all FIFOs, i can use this mechnism to shedule the packets in different queues, are there are any difference? Thank you very much! Regards, Zhenyu Wu
quoted hunk
I may be wrong on this, but I believe that RED can be attached to any queueing system, including the basic FIFO queues. In a sense, you're still using a scheduling system, when using the default arrangement, it's just a first-come, first-served one. RED is classless and applies to the whole of a queue. What that queue is attached to, if I understand it correctly, isn't important. It can be a class, but it can just as easily be everything going through that device. Again, someone correct me if I'm wrong, but as I understand it, there are four levels to the whole QoS/diffserv concept. One of these levels is the queueing discipline. This can be something like CBQ, WFQ, FIFO, PRIO, or whatever. This is how the data is organized, it does not describe how the data is sent. In the case of something like CBQ, you have a defined set of queues in parallel, with rules as to what packets fall into what queue. On the other hand, queueing schemes such as FIFO are flat. There's a single queue that everything goes through, though there may be different rules for how things get pushed to it. Another level is the scheduling mechanism. This describes how the data is sent, once organized, but does not describe the organization itself. If you've only one queue, then there's really not much to schedule. If you've multiple queues, then it's fairly normal to use "round robin" or "weighted round robin" to pick which queue to pull a packet from. Linux' CBQ uses "weighted round robin", according to the C file. The next level is the packet dropping mechanism. When queues flood, packets are going to be dropped. There's nowhere to store them. I'm pretty sure the default behaviour is to simply continue accepting packets, but to drop any that expire before being sent or which fall off the end of the queue (if the queue is bounded). RED, GRED, and a whole host of similar mechanisms, try to drop packets in a more controlled manner. However, that is really all they do. Finally, there are mechanisms for damping overly active applications, such as ECN. The idea here is that if you throttle back whatever is generating excess traffic, you don't get the problems assoicated with dealing with it. The "default" behaviour is to do nothing. When setting up QoS - on Linux or anything else - you basically pick one of each of the four categories to assemble a packet delivery system. Even without QoS, you're doing that, you're just using the defaults in all cases. The mechanisms are still going to be there. The Linux configuration menu does NOT match the above terminology, or the terminology in the source code. Thus, the source code identifies CBQ as a queueing discipline, but the configuration menu calls it a scheduler. The QoS help is also not very helpful, as it mostly tells people to look at the source. However, if you look at the source for CBQ or RED, for example, the explanation is relative to the cited papers, so you then have to go and read those before coming back and doing anything. This is one area I hope is going to get resolved in the reasonably near future. If not, I might have to come up with a patch myself. The very thought of that should send shivers down the spines of any kernel developers out there. Jonathan--- Zhenyu Wu <y030729@njupt.edu.cn> wrote:quoted
Thank you very much, i will try to find these papers which must be very helpful for me. The "more" means that whether there are other mechanisms not only for Linux. Sorry, i have not make it clear! Sometimes, i wonder whether the qdiscs such as CBQ, RED, GRED ... are belong to the scheduler mechanisms in linux enviroment. For example, In Red, which i can find are enqueue, and dequeue.... so, if i add a RED qidsc to a class, must i add a scheduler mechanism so that i can decide which packet in the queues will be scheduled and put to the link? Good luck, Best,__________________________________ Do you Yahoo!? The all-new My Yahoo! - What will yours do? http://my.yahoo.com
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