this smells a lot as a bad interaction between default window
size and mtu -- loopback has 16k default, maybe tar uses a
smallish window (32k is default now for net.inet.tcp.sendspace,
but used to be 16k at the time), which means only 1 or 2 packets in
flight at once, meaning that many times you get the 200ms delay
and your throughput goes way down.
cheers
luigi
On Tue, Oct 15, 2002 at 05:25:42PM -0700, Paul Herman wrote:
...
> Aha! Another victim who is willing to take a look at this! :-)
>
> It's an issue that was left unresolved in kern/24645. Bruce Evans
> brought this to my attention back during the unrelated "I have
> delayed ACK problems" thread on -net in January of 2001 and I then
> passed it on to jlemon. If you need a packet trace, let me know,
> but you should be able to reproduce it yourself. Even today on my
> 4.7-PRERELEASE I still get:
>
> mammoth# sysctl net.inet.tcp.delayed_ack=0
> net.inet.tcp.delayed_ack: 1 -> 0
> mammoth# time tar cf 127.0.0.1:/tmp/foo /kernel
> 0.000u 0.041s 0:00.33 12.1% 350+300k 0+0io 0pf+0w
>
> mammoth# sysctl net.inet.tcp.delayed_ack=1
> net.inet.tcp.delayed_ack: 0 -> 1
> mammoth# time tar cf 127.0.0.1:/tmp/foo /kernel
> 0.014u 0.033s 0:45.90 0.0% 700+600k 0+0io 0pf+0w
> ^^^^^^^
>
> It seems that lowering lo0 mtu to 1500 makes this particular
> problem go away. The magic mtu size is 2100. This makes me think
> that this is a big problem across GigE using 8K jumbo frames, not
> sure. Also, taring over the IPv6 lo0 interface seems to work OK.
>
> No idea what causes this.
>
> -Paul.
>
>
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