jamal wrote:

On Mon, 2007-11-06 at 16:49 +0200, Patrick McHardy wrote:

quoted

We have n empty HW queues with a maximum length of m packets per queue:

[0] empty
[1] empty
[2] empty
..
[n-1] empty

Asumming 0 i take it is higher prio than n-1.

Yes.

quoted

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:

n-1 (i think?)

Right.

quoted

[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. 

ok, so 0 is lower prio than n-1 

Higher priority. But we don't know what the priority of the
next packet is going to be, so we have to stop the entire
qdisc anyway.

quoted

To start it again at least one packet of queue n - 1 needs to be sent, 

following so far ...

quoted

which (assuming
strict priority) requires that queues 1 to n - 2 are serviced
first. 

Ok, so let me revert that; 0 is higher prio than n-1.

Yes.

quoted

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. 

"possibly long time" is where we diverge ;->

Worst cast is (n - 2) * (m - 1) + 1 full sized packet transmission
times.

You can do the math yourself, but we're talking about potentially
a lot of packets.

If you throw the burden to the driver (as i am recommending in all my
arguements so far), it should open up sooner based on priorities.
I didnt wanna bring this earlier because it may take the discussion in
the wrong direction. 
So in your example if n-1 shuts down the driver, then it is upto to the
driver to open it up if any higher prio packet makes it out.

How could it do that? n-1 is still completely full and you don't
know what the next packet is going to be. Are you proposing to
simply throw the packet away in the driver even though its within
the configured limits of the qdisc?