ridiculously slow VM memory performance on Ryzen CPU

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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Hi,

When testing IO heavy work on my VM backed by Ryzen 1700 CPU, I turned
to brd modules, but surprisingly, the speed is even slower than some HDD:

---
$ sudo modprobe brd rd_nr=1 rd_size=1048576
$ dd if=/dev/zero of=/dev/ram0 bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 9.9928 s, 107 MB/s
---
107MB is pretty lame...
Even some HDD could be faster than this.

On host, it's much better:
---
$ if=/dev/zero of=/dev/ram0 bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.754641 s, 1.4 GB/s
---

For host hardware:
CPU: Ryzen 1700 All cores @ 3.8G
Mem: DDR4 2400 dual channel (8G x 2)

For host software:
Kernel:       4.16.3-1-ARCH
Qemu:         2.11.1-2
Distribution: Archlinux

VM setup is mostly default setup done by libvirt.

I'm not sure if this is related to this bug:
https://www.redhat.com/archives/vfio-users/2017-April/msg00019.html

Thanks,
Qu


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