[LARTC] The effects of queueing on delay...(TX Ring Buffer the problem)

[Andy Furniss]

Jonathan Lynch wrote:
> This was down to the tx buffer size on the network card i was using. It
> was an Intel 82547EI gigabit Card using the e1000 driver and operating
> at 100mbit. The tx buffer was set to 256 which caused this huge delay.
> The minimum the driver lets me reduce the tx buffer size using ethtool
> is 80. By reducing the tx ring buffer to 80, the delay when there is
> full link utilisation and a maximum queue of 10 packets was reduced from
> 30ms to 10ms.
> 
> The 3com 3c59x vortex driver uses a tx buffer of 16. I reduced the tx to
> 16 on the e1000 driver, but the max throughput i could achieve on the
> interface went down.
> 
> Has anyone experimented with reducing the size of the tx buffer on this
> card to get a good balance between delay and throughput ?

Strange - I thought that as long as you are under rate for the link then 
the most htb should burst per tick is the burst size specified.

That assumes one bulk class - more will make it worse.

Andy.