Re: [Qemu-devel] ridiculously slow VM memory performance on Ryzen CPU

[Qu Wenruo]

On 2018年04月25日 21:18, Daniel P. Berrangé wrote:
> On Wed, Apr 25, 2018 at 05:48:42PM +0800, Qu Wenruo wrote:
>> 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.
> 
> You are sure that QEMU is actually using KVM, and not TCG emulation
> mode, as that could account for such terrible numbers. The QEMU
> argv would confirm this or can query it live with
> 
>    virsh qemu-monitor-command --hmp $GUESTNAME "info kvm"
> 
>> 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.
> 
> Ideally please share the guest XML description from 'virsh dumpxml $GUESTNAME'
> and the corresponding QEMU argv from /var/log/libvirt/qemu/$GUESTNAME.log

It turns out that my debugging config (KASAN + lockdep, maybe something
else) is causing the problem.

With lockdep only, the guest degrades to about 50% of host memory
performance.
With lockde + KASAN, only 10% of host.

So, in short, I'm just an idiot.
Sorry for the trouble.

Thanks,
Qu

> 
> Regards,
> Daniel
>

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