[Qemu-discuss] 100% Host CPU usage while guest idling

[Gareth Bryan]

Hi,

We have an appliance that runs a FreeBSD guest on a Yocto-based host
via qemu-system-x86_64.
Everything functions fine however the host uses n00% of the CPU (where
n = #smp) and RAM allocated to it whilst the 1 guest is sat nearing
idle.

Host:
PID     USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
4406    root      20   0 16.7g  16g  26m S  500 53.0  17958:38 qemu-system-x86

Guest:
CPU 0:  0.0% user,  0.0% nice,  0.4% system,  0.0% interrupt, 99.6% idle
CPU 1:  0.0% user,  0.0% nice,  0.4% system,  0.0% interrupt, 99.6% idle
CPU 2:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 3:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 4:  0.4% user,  0.0% nice,  0.0% system,  0.0% interrupt, 99.6% idle
Mem: 43M Active, 4783M Inact, 1530M Wired, 911M Buf, 9553M Free
Swap: 3072M Total, 3072M Free

I have logged this with the appliance vendor and received the response:
"This is expected behaviour and you will see the same in any case
where a Guest OS runs over a Host OS.
Host here has 5 CPUs and it has assigned all of them to Guest. Since
the Host is not being shared by any Guest OS; you will always see the
500% (or the 5 CPUs) given to qemu-system-x86.
I do see the same in lab and is very much expected"

This feels fundamentally wrong to me - If am I right in thinking this
I would appreciate a quick reply from a maintainer that I could use to
push back on the vendor to actually investigate
this further.

Versions below.

Many thanks,
Gareth

Host:
Linux 204a-node 3.10.100-ovp-rt110-WR6.0.0.31_preempt-rt #1 SMP Fri
Aug 3 01:59:01 PDT 2018 x86_64 x86_64 x86_64 GNU/Linux

Qemu:
QEMU emulator version 1.7.2, Copyright (c) 2003-2008 Fabrice Bellard

Command:
(Vendor identifying information has been removed)

/usr/bin/qemu-system-x86_64 \
-name REMOVED \
-S \
-machine pc-i440fx-1.7,accel=kvm,usb=off \
-m 16384 \
-realtime mlock=on \
-smp 5,sockets=5,cores=1,threads=1 \
-uuid 76277b29-3bd4-4dd4-a705-ed34d6449d6d \
-nographic \
-no-user-config \
-nodefaults \
-chardev 
socket,id=charmonitor,path=/var/lib/libvirt/qemu/REMOVED.monitor,server,nowait
\
-mon chardev=charmonitor,id=monitor,mode=control \
-rtc base=utc \
-no-shutdown \
-boot strict=on \
-device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 \
-device virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x17 \
-netdev tap,fd=22,id=hostnet0,vhost=on,vhostfd=23 \
-device virtio-net-pci,netdev=hostnet0,id=net0,mac=REMOVED,bus=pci.0,addr=0x11 \
-netdev 
tap,ifname=tap1,script=/etc/vehostd/XXX-em3-ifup,id=hostnet1,vhost=on,vhostfd=24
\
-device virtio-net-pci,netdev=hostnet1,id=net1,mac=REMOVED,bus=pci.0,addr=0x12 \
-netdev 
tap,ifname=tap2,script=/etc/vehostd/REMOVED-em4-ifup-SUMMIT,id=hostnet2,vhost=on,vhostfd=25
\
-device virtio-net-pci,netdev=hostnet2,id=net2,mac=REMOVED,bus=pci.0,addr=0x1c \
-netdev 
tap,ifname=tap3,script=/etc/vehostd/REMOVED-em4-re-re-ifup,id=hostnet3,vhost=on,vhostfd=26
\
-device virtio-net-pci,netdev=hostnet3,id=net3,mac=REMOVED,bus=pci.0,addr=0x1d \
-chardev pty,id=charserial0 \
-device isa-serial,chardev=charserial0,id=serial0 \
-chardev tty,id=charserial1,path=/dev/ttyS1 \
-device isa-serial,chardev=charserial1,id=serial1 \
-chardev tty,id=charserial2,path=/dev/ttyS2 \
-device isa-serial,chardev=charserial2,id=serial2 \
-chardev tty,id=charserial3,path=/dev/ttyS3 \
-device isa-serial,chardev=charserial3,id=serial3 \
-device i6300esb,id=watchdog0,bus=pci.0,addr=0x10 \
-watchdog-action reset \
-object rng-random,id=rng0,filename=/dev/random \
-device virtio-rng-pci,rng=rng0,max-bytes=1024,period=2000,bus=pci.0,addr=0x1e \
-smbios type=0,vendor="INSYDE
Corp.",version=REMOVED,date=11/03/2017,release=1.00 \
-smbios 
type=1,manufacturer=REMOVED,product=REMOVED,version=REMOVED,serial=VF-NET
\
-device REMOVED-pci,host=0000:1c:00.0 \
-device kvm-pci-assign,host=0000:00:14.0 \
-device pci-hgcommdev,vmindex=0,bus=pci.0,addr=0x16 \
-drive 
file=/REMOVED/REMOVED-current.img,if=none,id=drive-virtio-disk0,format=raw,cache=directsync,aio=native
\
-device 
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x13,drive=drive-virtio-disk0,id=virtio-disk0,config-wce=off,x-data-plane=on,bootindex=1
\
-drive 
file=/REMOVED/REMOVED-var-config.img,if=none,id=drive-virtio-disk1,format=raw,cache=directsync,aio=native
\
-device 
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x15,drive=drive-virtio-disk1,id=virtio-disk1,config-wce=off,x-data-plane=on,bootindex=-1
\
-drive 
file=/REMOVED/REMOVED-aux-disk.img,if=none,id=drive-ide0-0-1,format=raw,cache=directsync,discard=unmap
\
-device ide-hd,bus=ide.0,unit=1,drive=drive-ide0-0-1,id=ide0-0-1,bootindex=-1 \
-drive 
file=/REMOVED/images/0/REMOVED-platform.img,if=none,id=drive-ide1-0-1,format=raw,cache=directsync,discard=unmap
\
-device ide-hd,bus=ide.1,unit=1,drive=drive-ide1-0-1,id=ide1-0-1,bootindex=-1 \
-msg timestamp=on