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Re: Giving priority to messages

To: Andi Kleen <ak@xxxxxx>
Subject: Re: Giving priority to messages
From: Donald Becker <becker@xxxxxxxxx>
Date: Tue, 19 Sep 2000 22:25:36 -0400 (EDT)
Cc: Ramón Agüero <ramon@xxxxxxxxxxxxxxx>, netdev@xxxxxxxxxxx
In-reply-to: <20000920024319.A4744@xxxxxxxxxxx>
Sender: owner-netdev@xxxxxxxxxxx
On Wed, 20 Sep 2000, Andi Kleen wrote:

> You can set skb->priority to values between 0 and 15 
> (see include/linux/pkt_sched.h)
> What actually happens depends on the queueing discipline that is set for
> the interface. The default one uses a 3 way pfifo, which maps 6 and 7
> to the highest priority queue.
> 
> Usually this scheduling does not help much though, because the underlying
> devices have a longer queue itself where the priorities are ignored
> (e.g. bus mastering ethernet cards with big TX rings or worse a modem
> that does v.42bis) 

This is one of the reasons for drivers to keep their Tx queues to a
reasonable length.  Two years ago an almost-always-sufficient Tx queue
length was between 6 and 10, so most of my bus-master drivers use a queue
length of 10.  Ten 1500 byte packets at 10Mbps is still not too long, but
ten 60 byte packets (approx. 96 byte periods on the wire) isn't very long at
100Mbps.

If you care *that* *much* about priority and scheduling, you are probably
wrong about what you want.  We did some work on utilizing the real-time
transmit scheduling in the 3Com hardware, and found out that it made almost
zero difference to [[the multimedia application we were testing]].

Donald Becker                           becker@xxxxxxxxx
Scyld Computing Corporation             http://www.scyld.com
410 Severn Ave. Suite 210               Beowulf-II Cluster Distribution
Annapolis MD 21403


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