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Date:	Mon, 27 Dec 2010 15:43:46 +0200
From:	Pasi Kärkkäinen <pasik@....fi>
To:	Alkis Georgopoulos <alkisg@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Bypass flow control problems

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? 

-- Pasi

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