On Fri, 4 Jan 2002, Juha Saarinen wrote:
>
> > Did you fiddle yours ? As you can see, my lo adapter has a much smaller
> > MTU.. from a default slackware 7.1 install. I also note that your one
> > has a lot more activity than mine :)
>
> Nope... ~16KB seems to be the default. Checked two other RHL boxes, plus
> two FreeBSD 4.x systems which have the same. BSDi 4.1 uses 4,352 bytes
> though.

If you go into /usr/src/linux/drivers/loopback.c you'll see
that the MTU of your loopback device is hard-coded. YMMV!
on my SuSE 7.1 box running 2.2.18, it's 3924 but on 7.3 with
a lovingly hand-crafted 2.4.16 (with PPPoATM bits added),
it's 16KB or --

dev->mtu = (16 * 1024) + 20 + 20 + 12;

They've been gradually increasing the size with successive
kernel versions. It's tuned so that when the kernel allocates
space for the loopback pseudodevice's data in memory, so it
all *just* fits into one page. Too large, you thrash your disk.
routing. Too small, you're making too many calls to kmalloc.

Read The Source, Luke!

Cheryl

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