Re: howto increase MTU size?

To: Joseph Mack <mack.joseph@xxxxxxx>
Subject: Re: howto increase MTU size?
Cc: "lvs-users@xxxxxxxxxxxxxxxxxxxxxx" <lvs-users@xxxxxxxxxxxxxxxxxxxxxx>, Horms <horms@xxxxxxxxxxxx>, Roberto Nibali <ratz@xxxxxx>, <jmack@xxxxxxxx>
From: Julian Anastasov <ja@xxxxxx>
Date: Thu, 25 Apr 2002 22:53:45 +0000 (GMT)

On Thu, 25 Apr 2002, Joseph Mack wrote:

> according to
> the MTU of 1500 was chosen for 10Mbps ethernet and was kept
> for 100Mbps and 1Gbps ethernet for backwards compatibility
> on mixed networks. However MTU=1500 is too small
> for 100Mbps and 1Gbps ethernet. In Gbps ethernet jumbo frames
> (ie bigger MTU) is used to increase throughput.
> On my LVS performance page I found that decreasing the MTU
> decreased max throughput on 100Mbps ethernet. Since the decrease
> was approx linear with MTU, I wondered if the linearity would
> still be linear beyond 1500, and if increasing the MTU
> on 100Mbps would increase max throughput (ignoring
> the problem of fragmenting packets once they got off the LAN).

        Yes, with jumbo frames you can get some percents performance 
increase, may be different for the different NICs.

> I couldn't increase the MTU above 1500 with ifconfig or ip link.
> I found that the MTU seemed to be defined in
> linux/include/if_ether.h
> as
> and increased these by 1500, recompiled the kernel and net-tools
> and rebooted. I still can't install a device with MTU>1500

        By default, the 100mbit NICs allow MTU 68..1500. The GEs
allow longer MTU (up to 9000?). May be you see this error
on 100mbit NIC? Usually, each driver can control the MTU and
its limits.

> Any suggestions?
> Thanks
> Joe


Julian Anastasov <ja@xxxxxx>

<Prev in Thread] Current Thread [Next in Thread>