Interrupt Assignment on host

Interrupt Assignment on host

[Erik Rull]

Hi all,

after several tests it seems that the interrupt assignment on the host
systems influences the kvm performance drastically. If eth0, video and usb
are on seperate interrupts the perfomance is way better than if they are
shared (that means on the same one).

Normally this should not matter. But why is it within kvm? Does kvm somehow
hook up into the interrupt service routines?
My guest is Windows XP SP3.

Best regards,

Erik

Re: Interrupt Assignment on host

[Avi Kivity]

On 10/28/2009 11:16 PM, Erik Rull wrote:
> Hi all,
>
> after several tests it seems that the interrupt assignment on the host
> systems influences the kvm performance drastically. If eth0, video and 
> usb
> are on seperate interrupts the perfomance is way better than if they are
> shared (that means on the same one).
>
> Normally this should not matter. But why is it within kvm? Does kvm 
> somehow
> hook up into the interrupt service routines?

No.

> My guest is Windows XP SP3.

Can you describe how performance suffers?

Please provide vmstat (for host int/sec) and "kvm_stat  -l -f 
'exits|irq_exits'" output.

-- 
error compiling committee.c: too many arguments to function

Re: Interrupt Assignment on host

[Erik Rull]

Avi Kivity wrote:
> On 10/28/2009 11:16 PM, Erik Rull wrote:
>> Hi all,
>>
>> after several tests it seems that the interrupt assignment on the host
>> systems influences the kvm performance drastically. If eth0, video and 
>> usb
>> are on seperate interrupts the perfomance is way better than if they are
>> shared (that means on the same one).
>>
>> Normally this should not matter. But why is it within kvm? Does kvm 
>> somehow
>> hook up into the interrupt service routines?
> 
> No.
> 
>> My guest is Windows XP SP3.
> 
> Can you describe how performance suffers?
> 
> Please provide vmstat (for host int/sec) and "kvm_stat  -l -f 
> 'exits|irq_exits'" output.
> 

I'm sorry, but my target system has no python installed. Any ideas how to 
do that manually?

- Erik

Re: Interrupt Assignment on host

[Avi Kivity]

[-- Attachment #1: Type: text/plain, Size: 438 bytes --]

On 11/05/2009 09:09 PM, Erik Rull wrote:
>> Can you describe how performance suffers?
>>
>> Please provide vmstat (for host int/sec) and "kvm_stat  -l -f 
>> 'exits|irq_exits'" output.
>>
>
>
> I'm sorry, but my target system has no python installed. Any ideas how 
> to do that manually?

Try the attached bash script.  You'll want debugfs mounted on 
/sys/kernel/debug.

-- 
error compiling committee.c: too many arguments to function


[-- Attachment #2: kvm_stat.bash --]
[-- Type: text/plain, Size: 352 bytes --]

#!/bin/bash

p="/sys/kernel/debug/kvm"

old_exits=0
old_irq_exits=0

while :; do
    exits="$(cat "$p/exits")"
    irq_exits="$(cat "$p/irq_exits")"
    d_exits=$((exits - old_exits))
    d_irq_exits=$((irq_exits - old_irq_exits))
    printf "%10d %10d\n" "$d_exits" "$d_irq_exits"
    sleep 1
    old_exits="$exits"
    old_irq_exits="$irq_exits"
done