Re: network queue lengths

[David Young <dyoung%pobox.com@localhost>]

On Thu, Mar 19, 2009 at 11:43:23PM +0200, Mihai Chelaru wrote:
> Hi,
>
> David Young wrote:
>> I am concerned that the lengthy Tx queues/rings in NetBSD lead
>> to lengthy delays and unfairness in typical home/office gateway
>> applications.  What do you think?
>>  
>
> Uff, I was hoping for a moment that you want to write another packet  
> scheduler to replace the ancient ALTQ that looks mostly unmaintained :)
>
>> If a NetBSD box is the Internet gateway for several 10-, 100-, or
>> 1000-megabit clients, and the clients share a 1-, 2-, or 3-megabit
>> Internet pipe, it is easy for some outbound stream to fill both the
>> Tx ring (max 64 packets) and the output queues (max 256 packets) to
>> capacity with full-size (Ethernet MTU) packets.  Once the ring + queue
>> capacity is reached, every additional packet of outbound traffic that
>> the LAN offers will linger in the gateway between 1.3 and 3.8 seconds.
>>
>> Now, suppose that we shorten the interface queue, or else we "shape"
>> traffic using ALTQ.  Outbound traffic nevertheless spends 1/4 to 3/4
>> second on the Tx ring, which may defeat ALTQ prioritization in some
>> instances.
>
> It's not very clear to me how did you get this numbers. Transmitting 320  
> full-sized frames shouldn't take more than 50ms over 100Mbit full-duplex  
> ethernet link. Are you talking about some other in-kernel delays ?

The situation that I am talking about is a LAN "funnelled" into a narrow
WAN pipe.  The WAN pipe is 1-, 2-, or 3-megabit; the LAN is 100- or
1000-megabit.

Dave

-- 
David Young             OJC Technologies
dyoung%ojctech.com@localhost      Urbana, IL * (217) 278-3933