Re: [LARTC] Question on prio qdisc

[Lars Landmark <larslan@solan.zapto.org>]

On Wed, 2 Jul 2003, Cain, Joseph wrote:

> 
> I have been doing some experiments with the 
> prio qdisc. I haven't tried to understand the
> code, but the behavior I have observed in the
> experiments leads me to believe that the prio
> qdisc operates in the following manner when the
> interface queue is full:
> 1) Suppose that we are operating with the default
> 3 priority bands and the interface queue is full
> of packets of the lowest priority (indicated by the
> TOS byte).
> 2) Then suppose a new packet of highest priority is to be 
> sent to the interface for queuing.
> 3) It appears to me that the newly arriving packet of 
> highest priority will be dropped because the queue is
> full. Prio does not appear to drop one of the lower priority
> packets waiting in the queue to make room for the 
> higher priority packet.
> 
> The evidence for this conclusion is that I can set the
> "txqueuelen" to a large value to make ample room for 
> queuing high priority packets. Then if the interface 
> is sent a mixture of an overload of low priority
> packets and a small load of high priority packets, the
> high priority packet suffer just as high a packet loss
> percentage as the low priority packets. To me this indicates
> that they are getting dropped at the interface because the
> queue is full.

In prio_init(), a FIFO queue is establish for each of these 
three bands.  I thought that each of these FIFO queues could occupy 
txqueuelen packages.  According to you, this is not true.  however if your 
assumption is right, and you want to achieve dropping low priority package 
if high priority package is entering, then you could just make a small 
extension to the prio_enqueue code in linux/net/sched/sch_prio.c.   

Class/band 0 has highest priority. If you unsuccessfully try to enqueue a 
package to this class, you could make a call to prio_drop(). 
This function drop the package from lowest priority class, that has a package.
When returning from this function, there should be enough place to your high 
priority package.  Try now to enqueue this package once more, 
and you should be done.  Hopefully this will work. But it will work only if 
Linux only count packages and do not count how many bytes each of these 
FIFO queue holds.


Lars

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