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.
Okay, fair enough. :-)
Btw, I am *hopeful* that one day in the future Nemo will provide buffer
management on behalf of drivers. This will address some of the
long-standing races with "loan-up", and free drivers from making poor
decisions as to when to bcopy or use loan up. (Or maybe just allocate a
new DMA or DVMA buffer....)
-- Garrett
Drew
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