[brussels-dev] Re: [driver-discuss] Re: [networking-discuss] Brussels high-level design document

[Darren.Reed at Sun.COM]

Andrew Gallatin wrote:

>Garrett D'Amore writes:
> > The problem here is that the only reason to lower the MTU is to deal 
> > with cases where Path MTU discovery fails.  For example, lowering the 
> > MTU because your upstream provider doesn't properly deal with frames 
> > larger than a PPP size or somesuch.
> > 
> > Its frustrating that these cases still exist, but they do.  In general, 
> > I agree, that lowering the MTU should not be necessary.  And indeed, 
> > frankly nobody should need to touch the values provided by the media 
> > drivers when everything works properly.
>
>You may want to touch the values in order to reduce memory useage if
>you know you cannot use jubmo frames.  Since most drivers manage their
>own receive buffers, this can add up.  For example, my 10GbE driver,
>depending on load, may allocate up to a (tunable) maximum of 4096
>receive buffers.  The difference between 4096 1500b and 9000b frames
>is nearly 30MB.
>
>It would be nice if the driver could be notified that the MTU is
>changing so that it can re-allocate appropriately sized receive
>buffers.  Every other *nix that I've worked with does this.
>  
>

It would be nice if the buffer space used by a driver could be
specified in terms of MB rather than n buffers.

Darren