netdev - Re: Bypass flow control problems

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Date:	Tue, 28 Dec 2010 21:05:42 -0800
From:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>
To:	Pasi Kärkkäinen <pasik@....fi>
CC:	Alkis Georgopoulos <alkisg@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Bypass flow control problems

On Dec 27, 2010, at 8:43, Pasi Kärkkäinen <pasik@....fi> wrote:

> On Wed, Dec 22, 2010 at 07:51:31PM +0200, Alkis Georgopoulos wrote:
>> Hi,
>> 
>> I'm an IT teacher/LTSP developer. In LTSP, thin clients are netbooted
>> from a server and receive a lot of X and remote disk traffic from it.
>> 
>> Many installations have a gigabit NIC on the server, an unmanaged
>> gigabit switch, and 100 Mbps NICs on the clients.
>> 
>> With flow control on, the server is limited to sending 100 Mbps to all
>> the clients. So with 10 thin clients the server can concurrently send
>> only 10 Mbps to each one of them.
>> 
>> On NICs that support it, we turn flow control off and the server can
>> properly send 100 Mbps to each client, i.e. 1 Gbps to 10 clients.
>> 
>> * Is there any way to bypass that problem on NICs that do not support
>>   turning off flow control, like e.g. realteks?
>>   I.e. when a client sends a pause signal to the server, instead of the
>>   server pausing, to continue sending data to another client?
>>   Or even to limit the amound of data the server sends to each client,
>>   so that the clients never have to send pause signals?
>> 
> 
> You could set up QoS rules on the server to limit the network speed per client..
> 
>> * I really don't understand why flow control is enabled by default on
>>   NICs and switches. In which case does it help? As far as I
>>   understand, all it does is ruin gigabit => 100 Mbps connections...
>> 
>> * As a side note, since rtl8169 is a very common chipset, is there a
>>   way to disable flow control for that specific NIC?
>> 
>> This problem affects thousands of LTSP installations, we'd much
>> appreciate your knowledge and feedback on it.
>> 
> 
> Did you try disabling flow control from the switch? 

He stated the switch was a un-managed switch, so he would be unable to disable flow control on the switch.

>