[Click] Queues vs NIC tx/rx rings

[rchertov at purdue.edu]

Quoting rchertov at purdue.edu:

I meant to say Queue size of 100 instead of 50.

> Hello,
>    I have been trying to understand the effect of various queue (ring) sizes
> on
> the e1000 device driver and the click Queue element.  I use a click router
> to
> connect two nodes just by connecting the devices together so that the Click
> node acts as a layer 2 bridge and performs simple forwading.  As I
> understand
> from the Click paper, Click takes packets out of the RX ring of the driver
> and
> then passes it into the Queue element, assuming the config is
> Poll->Queue->ToDev.  Then the Queue puts the packet into the TX ring buffer
> of
> the device driver.  
> 
>    I have done some experiments on the 100 Mbit network where a UDP
> generator
> uses a raw socket to put the packets on the wire.   The generator reports
> how
> many times the send call was done successfully.  On the reciever I run
> Click/udpcount to count packets and the rate.  I have tried changing the
> tx/rx
> sizes via the ethtool as well as txqueuelen via ifconfig.  In all of the
> runs
> there does not seem to be much difference between the rates and packet
> counts. 
> Only difference occurs is when the click Queue is 50 or larger.  Then the
> send
> count == recieve count with the 148K pps rate.  In other cases the rate is
> about
> 148K pps but there are packet drops.  So I am curious as to why if I have
> large
> tx/rx buffers and a small Queue I get drops.  It would seem that Click would
> just need to quickly copy from the RX ring buffer to the TX ring buffer and
> there should not be much need for intermediate queueing. (or does the bus
> speed
> dictate a need for a large intermediate queue?)  Additionally if I have
> small
> (96) tx/rx rings but a Queue >= 50, then all the pakcets maket it through. 
> So I
> am curious in what cases the device driver ring sizes actually play any role?
>  
> 
> 
> Any insight would be much appreciated.
> 
> 
> Roman Chertov
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