[cfarm-users] Max UDP dgram size on M1 machine (gcc304)

Segher Boessenkool segher at kernel.crashing.org
Thu Mar 25 13:05:08 CET 2021

On Wed, Mar 24, 2021 at 05:30:03PM -0700, Kaz Kylheku wrote:
> On 2021-03-24 16:45, Segher Boessenkool wrote:
> >On Wed, Mar 24, 2021 at 08:39:10AM -0700, Kaz Kylheku via cfarm-users 
> >wrote:
> >The default is not a restriction, but meant for good performance:
> >it is 9216, which is the maximum MTU for any supported network
> >interface.
> I understand that sending large UDP packets over the network,
> even if IP fragmentation makes that work, is wrongheaded.
> However, the limit also affects local communication over loopback.
> The proof that loopback doesn't have that value as its MTU
> is that the datagrams are delivered over loopback when you increase
> the limit.

If you want your program to work over loopback only, why don't you use
much faster transport methods?

> Also:
> kaz at GCC-Farm-MiniMac txr % sysctl -a | grep dgram
> net.local.dgram.maxdgram: 2048
> net.local.dgram.recvspace: 4096
> net.inet.udp.maxdgram: 9216
> net.inet.raw.maxdgram: 8192
> What is "net.local.dgram.maxdgram": is that AF_UNIX?


> Is that also clipped to an even more stringent 2048 for performance,
> rather than as a restriction?

So that it will fit in one 4k page, presumably.  Again, no one cares
about datagram performance on loopback, it is a legacy feature.


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