[ kvm-Bugs-1906204 ] AMD NPT causes performance degradation

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Bugs item #1906204, was opened at 2008-03-03 13:45
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Technologov (technologov)
Assigned to: Nobody/Anonymous (nobody)
Summary: AMD NPT causes performance degradation

Initial Comment:
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.

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>Comment By: Avi Kivity (avik)
Date: 2008-08-27 14:41

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Make sure your host has the latest BIOS.

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Comment By: Marco Menardi (markit)
Date: 2008-08-26 22:59

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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

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Comment By: Anthony Liguori (aliguori)
Date: 2008-03-25 00:03

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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.


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Comment By: Technologov (technologov)
Date: 2008-03-03 17:25

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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.

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Comment By: Joerg Roedel (jroedel)
Date: 2008-03-03 16:40

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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?

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