2009/235 dladm Possible Values List

[Garrett D'Amore]

Michael Shapiro wrote:
> This case currently does not address the issue of how an administrator
> or layered software determines the optimal large MTU, as opposed to
> the maximum MTU.  The two are not always the same.  For example, on
> Neptune (nxge), the maximum is 9000, but the optimal large MTU is 8150,
> because of the size of the DMA transfers the card does in hardware.
> This case needs to address this issue explicitly, by either:
>
> (a) Defining an additional interface by which the optimal value
>     can be returned from the driver as another attribute, OR
>
> (b) Making extremely clear to driver writers that if a large MTU
>     size less than the maximum is more optimal than the maximum,
>     that the optimal size should be returned by this interface.
>
> My preference is for option (a), but others should weigh in.
>
> -Mike
>
>   
This feels like a "hardware tuning" element.

MTU configuration generally shouldn't need to worry about page sizes and 
such.  An extra DMA transfer is usually in the "noise" as far as 
overheads of network processing are concerned.

More specifically, large MTUs are intended, as I see it, to minimize the 
effect of per-packet overheads found in NIC hardware, switches, routers, 
and most especially *hosts*.  (I.e. the TCP/IP stack overheads.)   I 
suspect that because of these overheads, that in general the largest MTU 
you can configure is always "optimal", even if the hardware has to 
perform some extra DMA transfers to make them happen.

Unless some specific real world tests (e.g. TCP throughput or UDP stress 
tests) show otherwise, I'm disinclined to believe that there is any 
reason an administrator would need to know about the underlying hardware 
DMA limitations, or that the "optimum" value is anything other than the 
largest supported value.

    -- Garrett