[Bug 199727] CPU freezes in KVM guests during high IO load on host

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=199727

--- Comment #9 from Roland Kletzing (devzero@web.de) ---
https://qemu-devel.nongnu.narkive.com/I59Sm5TH/lock-contention-in-qemu
<snip>
I find the timeslice of vCPU thread in QEMU/KVM is unstable when there
are lots of read requests (for example, read 4KB each time (8GB in
total) from one file) from Guest OS. I also find that this phenomenon
may be caused by lock contention in QEMU layer. I find this problem
under following workload.
<snip>
Yes, there is a way to reduce jitter caused by the QEMU global mutex:

qemu -object iothread,id=iothread0 \
-drive if=none,id=drive0,file=test.img,format=raw,cache=none \
-device virtio-blk-pci,iothread=iothread0,drive=drive0

Now the ioeventfd and thread pool completions will be processed in
iothread0 instead of the QEMU main loop thread. This thread does not
take the QEMU global mutex so vcpu execution is not hindered.

This feature is called virtio-blk dataplane.
<snip>


i tried "virtio scsi single" with "aio=threads" and "iothread=1" in proxmox,
and after that, even with totally heavy read/write io inside 2 VMs (located on
the same spinning hdd on top of zfs lz4 + zstd dataset and qcow) and severe
write starvation (some ioping  >>30s), even while live migrating both vm disks
in parallel to another zfs dataset on the same hdd, i get absolutely NO jitter
in ping anymore. ping to both VMs is constantly at <0.2ms 

from the kvm pid:
-object iothread,id=iothread-virtioscsi0  
-device
virtio-scsi-pci,id=virtioscsi0,bus=pci.3,addr=0x1,iothread=iothread-virtioscsi0 
-drive
file=/hddpool/vms-files-lz4/images/116/vm-116-disk-3.qcow2,if=none,id=drive-scsi0,cache=writeback,aio=threads,format=qcow2,detect-zeroes=on 
-device
scsi-hd,bus=virtioscsi0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0,id=scsi0,bootindex=100

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