RE: [LARTC] Question on prio qdisc

[Leonardo Balliache <leoball@opalsoft.net>]

Hi, Joseph:

At 03:31 p.m. 08/07/03 -0400, you wrote:

>I understand your point, but I have tried to determine
>the answers to a couple of questions from the code
>without success (I don't understand the source code
>well enough). Maybe you could answer these:
>1) If I specify a queue length for the interface
>by configuring "txqueuelen" to some number (say 1000), then
>does that mean that the 3 FIFO queues for the 3 bands
>in either "prio" scheduler or the default "pfifo_fast" scheduler are set each
>to a size of 1000 packets? If this is the case, then I don't
>see how my high priority queue can drop packets when the high
>priority flow rate is well below what the link can support.
>Can you think of any reason this should happen?

Have a look to a previous e-mail I answered to Lars. Reason? What´s the 
entering rate? What's the departing rate? The queue will be filled to a 
rate defined by the difference between those rates.

>2) My testing leads me to believe that there are 3 queues,
>but the buffer allocated for a new packet enqueued to
>any of the queues is allocated from a common pool of buffers
>that might be of size equal to "txqueuelen" = 1000. Is it
>possible that it is implemented this way? I could see
>high priority packets being dropped if this was the
>implementation?
>******************************************

No, again, have a look to a previous e-mail I sent to Lars.

Best regards,

Leonardo Balliache

Here you have a copy of Lars' e-mail:

I'm not very sure if the total queue length is 100 or 300 because I was
searching the code and I can't find any information that tell me what the
real length is.

Because "ip link" shows me that qlen is 100, I have to suppose that total
length is 100.

But anyway, PRIO queing discipline principle calls that each priority queue
is independent, then we can't have the prio 0 queue full of prio 1 or 2
packets (being the filters working well). It goes against the PRIO queuing
discipline principle.

In "Differentiated Service on Linux HOWTO" (work in progress) I present
a PRIO queuing discipline explanation using some documentation taken
from Juniper Networks. Have a look at http://opalsoft.net/qos.

Also if you read about Cisco PQ the principle is identical. If we don't
respect the principle, then the queue can be any sort of queue, but
certainly, not a PRIO queue. Then it´s not posible to ask on PRIO for something
that drops an already enqueue packet to make run for a new arriving one.

I suggest that configurations and tests made to reach that conclusions were 
revised.



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