netdev - Query on Linux Kernel Network

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Message-ID: <20100727221555.GC17237@kumar.broadband.vsnl.com>
Date:	Wed, 28 Jul 2010 03:45:55 +0530
From:	Kumar Sanghvi <divinekumar@...il.com>
To:	netdev@...r.kernel.org
Subject: Query on Linux Kernel Network - Transmission Path

Hi All,

I have a query on the Transmission Path for the Linux Kernel Network
stack.
I understand that Qdisc/Traffic control can be used to prioritize
traffic on a per net_device basis.

However, I would like to know how to prioritize traffic between the
net_device(s) ?

Assume the below scenario:
-A Linux server has two net_device.
-One net_device -eth0, deals with IPv4 traffic over Ethernet
 This eth0 net_device is heavily loaded and there is a continuous
network activity going on.

-2nd net_device -ph0, deals with Phonet traffic.
 It occassionally transmits Voice traffic. It is required that traffic
over net_device ph0 should not suffer any delay.

Now as I understand, for the transmission path, the function
"__netif_schedule" will queue a net_device on the output_queue of
sofnet_data.
Also, the net_tx_action will pick up the net_device from output_queue
of softnet_data on last-in--first-out basis.

So, since eth0 has always IPv4 traffic to be sent, it will get queued
up continously in the output_queue of softnet_data. In this scenario,
what would happen to the ph0 device carrying the Phonet traffic? Will
the ph0 device get a chance to queue up on the output_queue of
softnet_data in the middle of continously coming IPv4 traffic from
eth0?
Or will the ph0 get starved in this case? Or the ph0 will suffer a
delay in getting scheduled for transmission?

In short, is there any way to prioritize the transmission among the
net_device eth0 and ph0 i.e. is it possible to queue up ph0 carrying
Phonet traffic on output_queue of softnet_data first in comparison to
eth0 carrying IPv4 data, so that ph0 traffic does not suffer any delay?

Pardon me if I have misunderstood anything or if my knowledge is
outdated in this regards.
I would really appreciate some pointers related to this priority among
net_device(s) for transmission.

Thanks & Regards,
Kumar.
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