Re: Changes in the network interface queueing handoff model
- From: John Polstra <jdp@xxxxxxxxxxx>
- Date: Mon, 31 Jul 2006 10:34:53 -0700 (PDT)
On 31-Jul-2006 Petri Helenius wrote:
Robert Watson wrote:
Actually most em cards support 4096 descriptors each way.
I tend to agree, but implemented full queueing support for if_em to
make sure I understood to complexity implications of completely
removing queueing from the ifnet side dispatch. I guess an
interesting question for us is how we decide what the right threshold
is to implement software queuing. Do any if_em cards need software
queueing, or do they all have adequate in-hardware queues as is?
Entirely cutting the queue code would significantly simplify
em_startmbuf.
Yes, even the earliest ones supported 4096 descriptors on paper. In
practice, the early chips had bugs that required the entire descriptor
ring to fit in a single page of memory. That limited them to 4096/16
= 256 transmit descriptors on x86 hardware at the time. That chip
bug was fixed a long time ago, though, and in any case 256 transmit
descriptors is a lot for most applications.
John
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