[vortex] QoS Linux Support /DPD1 Format

[Donald Becker]

On Tue, 4 Dec 2001, Bogdan Costescu wrote:
> From the doc file that I have, it looks like the sort of scheduling that 
> the 3Com cards support is not useful for all applications. In order for 
> the high priority packet to be inserted at the top of the queue, you have 
> to set the scheduling timer, meaning that the top packet is to be delayed 
> until the counter is exceeded.

Yes, you understand the problem.  The 3Com scheduling method doesn't
increase the priority of packets, it can only insert pauses into the
transmit process.

Most bandwidth-sensitive applications don't care about a transmit delay,
they only want their packets to arrive at the remote end before a
specific time.  That's why a "greedy" algorithm, as standard Ethernet
uses, is almost always the best approach.

> If one or more urgent packet(s) need(s) to be sent within this period,
> it/they can be sent immediately, however this also means that if no
> urgent packets arrive within this period, the whole "on hold" time is
> wasted. In other words, you add a guaranteed latency for each packet
> in the normal queue.

A good summary is "mixing bulk and urgent traffic at 100% utilization is
fundamentally hard".  When you commit to sending bulk traffic, you will
sometimes block urgent traffic.  In theory, you only need one restroom
stall for every 70-100 people, but you don't find anyone provisioning at
that proportion.  Networking is similar: it's cheaper and easier to just
over-provision than it is to carefully schedule.  ("Your schedule for 
today is slightly less than three minutes at 9:58, 2:03 and 5:43pm.  For
tomorrow it's 9:03, 9:45 and again 9:17pm".)

Donald Becker				becker@scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993