jamal wrote: > On Mon, 2007-11-06 at 16:03 +0200, Patrick McHardy wrote: > >>Read again what I wrote about the n > 2 case. Low priority queues might >>starve high priority queues when using a single queue state for a >>maximum of the time it takes to service n - 2 queues with max_qlen - 1 >>packets queued plus the time for a single packet. Thats assuming the >>worst case of n - 2 queues with max_qlen - 1 packets and the lowest >>priority queue full, so the queue is stopped until we can send at >>least one lowest priority packet, which requires to fully service >>all higher priority queues previously. > > > I didnt quiet follow the above - I will try retrieving reading your > other email to see if i can make sense of it.
Let me explain with some ASCII art :) We have n empty HW queues with a maximum length of m packets per queue: [0] empty [1] empty [2] empty .. [n-1] empty Now we receive m - 1 packets for each all priorities >= 1 and < n - 1, so we have: [0] empty [1] m - 1 packets [2] m - 1 packets .. [n-2] m - 1 packets [n] empty Since no queue is completely full, the queue is still active. Now we receive m packets of priorty n: [0] empty [1] m - 1 packets [2] m - 1 packets .. [n-2] m - 1 packets [n-1] m packets At this point the queue needs to be stopped since the highest priority queue is entirely full. To start it again at least one packet of queue n - 1 needs to be sent, which (assuming strict priority) requires that queues 1 to n - 2 are serviced first. So any prio 0 packets arriving during this period will sit in the qdisc and will not reach the device for a possibly quite long time. With multiple queue states we'd know that queue 0 can still take packets. >>Your basic assumption seems to be that the qdisc is still in charge >>of when packets get sent. This isn't the case if there is another >>scheduler after the qdisc and there is contention in the second >>queue. > > > My basic assumption is if you use the same scheduler in both the > hardware and qdisc, configured the same same number of queues and > mapped the same priorities then you dont need to make any changes > to the qdisc code. If i have a series of routers through which a packet > traveses to its destination with the same qos parameters i also achieve > the same results. Did my example above convince you that this is not the case? - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
