lots of APIC_TMICT exit on VM with assigned NIC

lots of APIC_TMICT exit on VM with assigned NIC

[sheng qiu]

Hi all,

i saw lots of APIC_TMICT exits while running a linux guest with
assigned NIC. The APIC_TMICT exits is only a little fewer than the
number of injected interrupts while handling network traffic on the
assigned NIC.
The network traffic was generated by running netperf on another
machine targeted at the VM.

Is this normal? if not, what kind of problem may cause it?

Thanks,
Sheng

Re: lots of APIC_TMICT exit on VM with assigned NIC

[Avi Kivity]

On 07/12/2012 02:54 AM, sheng qiu wrote:
> Hi all,
> 
> i saw lots of APIC_TMICT exits while running a linux guest with
> assigned NIC. The APIC_TMICT exits is only a little fewer than the
> number of injected interrupts while handling network traffic on the
> assigned NIC.
> The network traffic was generated by running netperf on another
> machine targeted at the VM.
> 
> Is this normal? if not, what kind of problem may cause it?

I'm guessing the driver sets a timer on every interrupt.

What is the value written?  If it's large, perhaps we can set some slack
on the timer so it's not re-set every time.


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

Re: lots of APIC_TMICT exit on VM with assigned NIC

[sheng qiu]

thanks for your reply.
the value is very large. is there anyway that i  can avoid such
frequent exits due to APIC_TMICT? do i need to modify some codes in
kvm kernel module or adapt some parameters?

Thanks,
Sheng

On Thu, Jul 12, 2012 at 3:50 AM, Avi Kivity <avi@redhat.com> wrote:
> On 07/12/2012 02:54 AM, sheng qiu wrote:
>> Hi all,
>>
>> i saw lots of APIC_TMICT exits while running a linux guest with
>> assigned NIC. The APIC_TMICT exits is only a little fewer than the
>> number of injected interrupts while handling network traffic on the
>> assigned NIC.
>> The network traffic was generated by running netperf on another
>> machine targeted at the VM.
>>
>> Is this normal? if not, what kind of problem may cause it?
>
> I'm guessing the driver sets a timer on every interrupt.
>
> What is the value written?  If it's large, perhaps we can set some slack
> on the timer so it's not re-set every time.
>
>
> --
> error compiling committee.c: too many arguments to function
>
>



-- 
Sheng Qiu
Texas A & M University
Room 332B Wisenbaker
email: herbert1984106@gmail.com
College Station, TX 77843-3259

Re: lots of APIC_TMICT exit on VM with assigned NIC

[Avi Kivity]

On 07/12/2012 08:32 PM, sheng qiu wrote:
> thanks for your reply.
> the value is very large. 

Can you provide samples?

If it's large and fairly constant it means a timer keeps being pushed
back by the interrupts/


> is there anyway that i  can avoid such
> frequent exits due to APIC_TMICT? do i need to modify some codes in
> kvm kernel module or adapt some parameters?
> 

No, a guest change is needed.  You need to find out what timer is
involved at modify it to set some slack, with
hrtimer_set_expires_range() or similar.
-- 
error compiling committee.c: too many arguments to function