Re: [Qemu-discuss] Qemu on Windows (with haxm and whpx), minimal Ubuntu, SLIRP and OpenVPN: very slow

[Michael Fritscher]

Am 13.05.2019 um 10:32:48 schrieb Michael Fritscher:
> Good day,
>
> Scenario:
> Host: Windows 7 and 10 64 bit, Skylake+ CPUs
> Qemu Version: 3.1 (4.0 isn't available yet on https://qemu.weilnetz.de/)
> Accelerators: HAXM and WHPX
> Guest: Ubuntu 18.04.2 64 bit, minimal install
> Guest's tasks: OpenVPN, iptables, tc
>
> Configuration:
> -machine q35,accel=kvm:hvf:whpx:hax:tcg
> -m 512
> -smp 1
> -rtc base=utc
> -drive file=layertc_environmentmanager_amsvm.iso,if=virtio,media=cdrom
> -vga std
> -netdev
>      user,id=natted,
>      restrict=n,
>      net=172.31.1.0/255.255.255.0,
>      dhcpstart=172.31.1.1,
>      host=172.31.1.3,
>      dns=172.31.1.4
> -device virtio-net,netdev=natted,mac=00:50:56:00:08:00
> -nodefaults
> -monitor tcp:localhost:4444
> -kernel vmlinuz
> -initrd initrd.img
> -append "boot=casper textonly quiet nosplash nofb fb=false pti=off spectre_v2=off 
> l1tf=off nospec_store_bypass_disable noprompt"
>
> (the iso is a casper livecd, but I start the linux kernel directly because of 
> boot speed)
>
> Problem:
> It is slow...
> While a openssl speed -elapsed -evp aes-128-cbc is ok (500MB/sec and beyond), 
> OpenVPN struggles.
> I get 70% virtual CPU usage , mostly on OpenVPN and system load while transfering 1-2 MB/s with a UDP video stream. The measured delay is also 
> rather spiky and is about 30...40 ms. On the host I get a cpu usage of about 10% (=almost a core) I see this both on the server and the client 
> side.
>
> On the player edition of the big commercial player, the results are _much_ 
> better (10% virtual CPU usage, delay of 4-6 ms)
>
> How can I further debug this? Do you know any knobs I can try? Slirp, while there are warnings of being slow, doesn't seem to be the culprit as 
> I can get easily >>10 MB/s through it. And I see the big CPU usage of OpenVPN
>
> Best regards,
> Michael Fritscher

Hi again,

after a bit of digging, openVPN seems to call read_hpet with about 1 kHz, which is, regarding to the program perf, the main culprit. Is a call 
rate of this frequency normal and qemu is just very slow to handle them, or is there another problem?

Best regards,
Michael Fritscher
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