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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