netdev - Re: Bypass flow control problems

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20101227134346.GD2754@reaktio.net>
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

--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html