[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 #8 from Roland Kletzing (devzero@web.de) ---
yes.

i found the following interesting information. i think this explains a LOT.

https://docs.openeuler.org/en/docs/20.03_LTS/docs/Virtualization/best-practices.html#i-o-thread-configuration

I/O Thread Configuration
Overview

By default, QEMU main threads handle backend VM read and write operations on
the KVM. This causes the following issues:

    VM I/O requests are processed by a QEMU main thread. Therefore, the
single-thread CPU usage becomes the bottleneck of VM I/O performance.
    The QEMU global lock (qemu_global_mutex) is used when VM I/O requests are
processed by the QEMU main thread. If the I/O processing takes a long time, the
QEMU main thread will occupy the global lock for a long time. As a result, the
VM vCPU cannot be scheduled properly, affecting the overall VM performance and
user experience.

You can configure the I/O thread attribute for the virtio-blk disk or
virtio-scsi controller. At the QEMU backend, an I/O thread is used to process
read and write requests of a virtual disk. The mapping relationship between the
I/O thread and the virtio-blk disk or virtio-scsi controller can be a
one-to-one relationship to minimize the impact on the QEMU main thread, enhance
the overall I/O performance of the VM, and improve user experience.
Configu

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