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Linux - Virtualization and Cloud This forum is for the discussion of all topics relating to Linux Virtualization and Linux Cloud platforms. Xen, KVM, OpenVZ, VirtualBox, VMware, Linux-VServer and all other Linux Virtualization platforms are welcome. OpenStack, CloudStack, ownCloud, Cloud Foundry, Eucalyptus, Nimbus, OpenNebula and all other Linux Cloud platforms are welcome. Note that questions relating solely to non-Linux OS's should be asked in the General forum.

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Old 02-05-2015, 06:01 AM   #1
Holering
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Qemu KVM vga passthrough slow cpu speed


Slackware 64 14.1 and Qemu 2.0.

I'm using vga passthrough for Windows 7 x64 guest and CPU performance is slow. Is this because I'm using the default 440FX chipset (machine type PC)? I'm using "host" for cpu type, and qemu-kvm.

Heard reports of 95% native performance possible but find it hard to believe. I think I'm getting around %40-%60 and maybe slower in some cases.

Last edited by Holering; 02-05-2015 at 09:27 AM.
 
Old 02-05-2015, 08:45 AM   #2
smallpond
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VGA passthrough requires an IOMMU and VT-d (Intel) or Vi (AMD) support to allow the guest VM to directly access your second graphics card. When using VGA passthrough the guest controls the actual card, not an emulated i440FX, so it will run at full speed. Instead of using -machine you will need to give the individual options to qemu including the PCI location of the VGA controller that you are passing through.

http://www.linux-kvm.org/page/How_to...th_VT-d_in_KVM
 
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Old 02-05-2015, 09:33 AM   #3
Holering
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Okay I will try without specifying -machine.

Other things I tried: Set host cpu governor to performance but it doesn't help. Also, when I run Windows 98 guest, I have a constant %100 cpu usage during idle with sluggish response. I thought maybe this had something with the watchdog timer?

EDIT:
Tried with hugetlbfs and 2048 hugepages (4GB guest ram). Didn't notice any improvement (still sluggish cpu). Also used cache=none for "Windows 7 x64" raw img file, but this slows down boot time quite a bit. I can't play HD movies without choppy performance.

Any help is greatly appreciated!

Last edited by Holering; 02-05-2015 at 12:34 PM.
 
Old 02-07-2015, 01:16 PM   #4
Holering
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Changed i/o scheduler to deadline, nested paging off, and virtual HD to vfio with cache=none. Still no improvement (maybe a hair better performance).

Also installed CPU scheduler patch for bulldozer CPU in Windows 7 x64, and disabled core parking in registry. Very minimal improvement but still terrible CPU use (hovers around %20 each core at most with demanding applications). I have a 7.4 CPU score in "Windows Experience" which doesn't seem bad IMO.

I don't understand why I get much better speeds natively? I hear claims of 95+% native performance with kvm, but this clearly isn't happening on my end. CPU consumption stays low and my GPU clearly becomes bottlenecked.

Forgot to mention I'm using piledriver with a Radeon HD 7750. Guest has 4gb RAM and 2048 hugetlbfs.
 
Old 02-08-2015, 04:55 PM   #5
Holering
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Still get bad performance.

Native Windows 7 x64 I try a FPS demo-game "Alien Rage". It gets 60FPS constant. In KVM it gets only around 30-40FPS no matter how low in-game performance settings are; also, cpu usage hovers around %25. Also tried the q35 chipset, and connecting vfio GPU to ioh3420 device (wasn't using ioh3420 before). How are others getting almost native speed https://www.youtube.com/watch?v=37D2bRsthfI?

Forgot to mention I'm using FX 6300 piledriver CPU. Guest "Windows 7 x64" has 4GB of ram.

Last edited by Holering; 02-08-2015 at 05:06 PM.
 
  


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