Re: Dom0 bad network performance

[Matthias Petermann <mp%petermann-it.de@localhost>]

Hello Jaromir,

Am 13.02.21 um 22:14 schrieb Jaromír Doleček:
> There is one other thing that I remembered from last time I was looking at this.
>
> IIRC it's just the network performance Dom0 <-> outside which was slow
> compared to native, for DomU (running on that Dom0) <-> outside it was
> line-speed nevertheless for me.
>
> Perhaps then this is a red herring, and not really worth investigating
> - you will not really be running anything directly on the Dom0.

If that were the case, then I would be able to live with it because I 
see it the same way as you - in Dom0, the network speed is not so 
important to me. However, even in the DomU's no better performance is 
retrievable - regardless of whether the DomU is a NetBSD 9.1 stable or a 
NetBSD current. My comparison measurement with iperf brought it to 
almost identical values around 25 MB/s for NetBSD 9.1 Stable. The values 
for NetBSD-current were noticeably lower at ~18 MB/s.

```
peterma@nuc:~> iperf3 -w 128K -c 192.168.2.51
Connecting to host 192.168.2.51, port 5201
[  5] local 192.168.2.40 port 35990 connected to 192.168.2.51 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  25.5 MBytes   214 Mbits/sec    0    164 KBytes 

[  5]   1.00-2.00   sec  26.8 MBytes   225 Mbits/sec    7    192 KBytes 

[  5]   2.00-3.00   sec  26.3 MBytes   221 Mbits/sec   26    151 KBytes
```

> There are a bunch of optimizations particularly in pmap, which are
> disabled on Dom0 kernel compared to the native one. It also needs to
> do MMU updates via Xen hypercalls. So it will always be somewhat
> slower than native.

Well, maybe my combination of low-end CPU and NetBSD 9.1 Dom0 without 
SMP is actually the decisive factor that makes the lack of optimizations 
particularly noticeable here. I still have a little hope that maybe it's 
just a setting thing in the network stack? During the test, the CPU is 
80% idle, so would have reserves.

Kind regards
Matthias