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[Xen-devel] Poor performance on HVM (kernbench)

To: "xen-devel mailing list" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Poor performance on HVM (kernbench)
From: "Todd Deshane" <deshantm@xxxxxxxxx>
Date: Wed, 10 Sep 2008 14:23:17 -0400
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Reply-to: deshantm@xxxxxxxxx
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Hi All,

We are continuing our Xen vs. KVM benchmarking that I presented at Xen summit.

This time, we are focusing on newer versions and also planning to
include Xen HVM
and KVM with PV drivers results. As well as adding some more tests.

I have setup Xen 3.3 from source, and am using Linux 2.6.27-rc4 for
all the guests.

Below are some raw kernbench results, which clearly show that I have a problem
with Xen HVM. It may just be a configuration issue, but we have tried
all that we
could think of so far (i.e file:, instead of tap:aio). I have also
tried xen-unstable and
it doesn't seem to produce any better results. I am also in the
process of trying
kernbench on older versions of Xen HVM.

here is the xm command line
xm create /dev/null name=benchvm0 memory=2048
kernel="/usr/lib/xen/boot/hvmloader" builder="hvm"
vnc=1 vncdisplay=0 vif=mac=AA:BB:CC:DD:EE:00,bridge=br0
vif=mac=AA:BB:CC:DD:EE:7b,bridge=br1 vncviewer="yes"
on_poweroff=destroy on_reboot=restart on_crash=preserve

I will also consider an IO test, such as iozone to see if
the disk IO problems are a cause. The dom0 cpu
doesn't seem to be under much load at all during the
kernbench run.

System time on the kernbench run is 1/2 of the time, so does
that suggest either disk IO or guest scheduling problem?

System time on the other cases is 1/4 or less on the other

If anybody has any ideas, suggestions, or can even run Xen HVM kernbench
vs. native on their setup to compare against that would be very helpful.

The system setup is a Intel core2 dual 4 GB of ram.
The HVM guest does run the libata driver similar to KVM with emulated drivers.


KVM PV drivers

Average Optimal load -j 4 Run (std deviation):
Elapsed Time 527.572 (0.681337)
User Time 404.3 (0.982141)
System Time 122.552 (0.468636)
Percent CPU 99 (0)
Context Switches 116020 (180.82)
Sleeps 31307 (94.2072)

KVM Emulated drivers

Average Optimal load -j 4 Run (std deviation):
Elapsed Time 527.968 (0.450744)
User Time 403.95 (0.342929)
System Time 122.134 (0.550709)
Percent CPU 99 (0)
Context Switches 115907 (214.3)
Sleeps 31302.4 (88.7175)

Xen PV

Average Optimal load -j 4 Run (std deviation):
Elapsed Time 446.876 (0.130115)
User Time 392.088 (0.339367)
System Time 54.76 (0.391088)
Percent CPU 99 (0)
Context Switches 64601.4 (163.314)
Sleeps 31214.8 (183.53)


Average Optimal load -j 4 Run (std deviation):
Elapsed Time 2081.71 (34.0459)
User Time 617.36 (3.61771)
System Time 1430.36 (28.3309)
Percent CPU 98 (0)
Context Switches 331843 (5283.28)
Sleeps 37329.8 (91.538)

KVM Native (Linux)

Average Optimal load -j 8 Run (std deviation):
Elapsed Time 216.076 (0.121778)
User Time 381.122 (0.259557)
System Time 43.242 (0.278783)
Percent CPU 196 (0)
Context Switches 75483.2 (389.988)
Sleeps 38078.8 (354.267)

Xen native dom0 kernel

Average Optimal load -j 8 Run (std deviation):
Elapsed Time 228.504 (0.0808084)
User Time 384.014 (0.657632)
System Time 64.028 (0.733669)
Percent CPU 195.8 (0.447214)
Context Switches 35270.4 (264.36)
Sleeps 39493.4 (266.222)

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