Eugene Grosbein <eu...@grosbein.net> wrote:
> 11.12.2017 2:54, Michael Grimm wrote:
>> *BUT* if I do boot with the default 1500 setting,
>> changing the MTU to e.g. 1450 and *immediately* back to 1500 manually,
>> I do not encounter any performance loss at all. Why?
>> Even when booting 1490 and immediately setting the MTU manually to 1500 I do
>> not see any performance loss. Strange.
>
> Interface MTU is used to assing 'mtu' attribute to corresponding route in the
> system routing table.
> Lowering interface MTU lowers route mtu, but raising interface MTU does *not*
> raises route mtu,
> use "route -n get" command to check it out. So, you still use low mtu really.
Bingo!
NEW> ifconfig vtnet0
vtnet0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu
1490
NEW> route -n get freebsd.org
...
recvpipe sendpipe ssthresh rtt,msec mtu weight expire
0 0 0 0 1490 1 0
NEW> ifconfig vtnet0 mtu 1500 up
NEW> ifconfig vtnet0
vtnet0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu
1500
NEW> route -n get spiegel.de
...
recvpipe sendpipe ssthresh rtt,msec mtu weight expire
0 0 0 0 1490 1 0
I didn't know that. And that explains all my observations.
>> Hmm, how would one check that? The output is to fast for me ;-) Seriously,
>> how should one check this?
>
> With your eyes :-) Use tcpdump -c flag to limit number of lines, redirect
> output to a file
> and carefully compare some packets using their ID that tcpshow shows.
Ok. I will do that at some later time ;-)
I'd like to thank you again for your input and with kind regards,
Michael
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