mtu

[Kerry Hoath]

The ethernet frame size is 1524 bytes. You can't
raise the mtu higher than that you just end up with a pile of back to back
full-sized ethernet frames and loose performance due to the
fragmentation.
What transferrs are you trying to speed up and what sort of card is this? There may
be other options you can tweak that don't involve fooling with the 
framesize on ethernet.
Increasing mtu can also cause problems with cards that are not capable of
receiving giant frames. You're also violating
the ethernet standards if you exceed the maximum framesize.
I doubt you will gain any performance by reducing wire overhead, your
need for speed can be solved in other ways. How fast is this machine and is the
card isa/pci?
If it isn't a pci busmaster and is an rtl8029, it's a cheap
card and is a connectivity solution. Snap it in half and go buy a real card
if speed and system performance is important to you or tolerate
the slower speeds to save money.
3com 905 cards cost a lot of money for a good reason, they are fast
efficient and easy on cpu.
The rtl8029 and rtl8139 are mass market pci ethernet chips that aren't known
for speed. Linux usually drives them as best it can but
there are all sorts of hardware limitations to work around and
bugs that bite.

Regards, Kerry.
On Tue, Mar 26, 2002 at 02:28:26PM -0500, Igor Gueths wrote:
> Hi Kerry. Wouldn't increasing the mtu value decrease over head? Because I am trying to raise the value to 10000 or something, not lower it. 
> ----- Original Message ----- 
> From: Kerry Hoath <kerry at gotss.net>
> To: <speakup at braille.uwo.ca>
> Sent: Tuesday, March 26, 2002 6:46 AM
> Subject: Re: mtu
> 
> 
> > I can't see why you'd want to do this on an ethernet
> > interface anyway since ethernet is tuned to handle mtu of 1500 and reducing it
> > is just going to increase wire overhead and do nothing more.
> > MTU discovery means that if something else has a lower mtu this will be communicated back
> > to the kernel and th e networking stack will take care of it.
> > Well just because I don't know why you want to do this doesn't mean it can't be done.
> > Once the route to the interface is in place you need to del the route and re-add it
> > with the mtu value so for example:
> > route del 192.168.1.0
> > route add -net 192.168.1.0 dev eth0 mtu 768
> > and I think the ip tools can do this too
> > try ip route help 
> > and
> > ip nei help
> > 
> > Regards, Kerry.
> > On Mon, Mar 25, 2002 at 08:19:13PM -0500, Igor Gueths wrote:
> > > Hi. Has anyone successfully changed their mtu value on their network interface? I am unable to do this via /etc/network/interfaces. I put mtu 1000 after the line where it says iface eth0 inet dhcp. I restarted inetd and init, and the value hasn't changed. Does anyone know how to go about doing this? Thanks! 
> > > 
> > > 
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> > Kerry Hoath:  kerry at gotss.net kerry at gotss.eu.org or  kerry at gotss.spice.net.au
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-- 
Kerry Hoath:  kerry at gotss.net kerry at gotss.eu.org or  kerry at gotss.spice.net.au
ICQ: 8226547 msn: kerry at gotss.net Yahoo: kerryhoath at yahoo.com.au