#230 AMD NPT causes performance degradation

[Technologov]

Platform: F7/x64, AMD Barcelona K10, KVM-61.

Guest: Windows XP SP2.

By default, the new Nested Page Tables is enabled, which should accelerate guests.

While it does accelerate guests in some areas, particularly guest OS setup time dropped by 20% - which is great, but in other areas I see performance degradation.

For example:
Passmark PerformanceTest v6.1 shows
2D Graphics Marks: 78.6 (without NPT)

2D Graphics Marks: 18.9 (with NPT)

NPT was disabled using:

rmmod kvm-amd

modprobe kvm-amd npt=0

dmesg | tail

and all the graphics feel more sluggish, and way slower.
I have used SDL rendering.

-Alexey, 03.03.2008.

[Joerg Roedel]

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Reloading the KVM-AMD module does not disable NPT completly. You have to reload the KVM base module too. Can you reproduce your numbers if you reload both and disable NPT?

[Technologov]

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Yes, those numbers are always reproducible, even with reloading KVM base module too.

Just try it for yourself, the benchmark is shareware (free for 30 days) - but VMs rarely live any longer anyways :)

link:
http://www.passmark.com/

-Alexey, 03.03.2008.

[Anthony Liguori]

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I am able to reproduce this although the results aren't as extreme. The biggest impact is coming from the passmark 2D Graphics Filled Rectangles test. Only about 40k exits are occurring while this benchmark is being run. It's not impacted by the use of large pages. Still investigating.

[Marco Menardi]

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Same here, XP has a noticeable slow down. Was faster with my AMD X2 that is with 4 core Phenom. WIth npt=0 is fast again (as with X2 or better). Tried with kvm 70...73, debian Sid 64 as host

[Avi Kivity]

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Make sure your host has the latest BIOS.

[Marco Menardi]

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Latest bios flashed, but still performances issues. Only with npt=0 is noticeable faster
(btw, tested with guest at res 1680x1050 or 1920x1200, at full screen or not)