Re: Strange CPU usage pattern in SMP guest

[Avi Kivity <avi@redhat.com>]

On 03/21/2010 04:55 PM, Sebastian Hetze wrote:
> On Sun, Mar 21, 2010 at 02:19:40PM +0200, Avi Kivity wrote:
>    
>> On 03/21/2010 02:02 PM, Sebastian Hetze wrote:
>>      
>>> 12:46:02     CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest   %idle
>>> 12:46:03     all    0,20   11,35   10,96    8,96    0,40    2,99    0,00    0,00   65,14
>>> 12:46:03       0    1,00   11,00    7,00   15,00    0,00    1,00    0,00    0,00   65,00
>>> 12:46:03       1    0,00    7,14    2,04    6,12    1,02   11,22    0,00    0,00   72,45
>>> 12:46:03       2    0,00   15,00    1,00   12,00    0,00    1,00    0,00    0,00   71,00
>>> 12:46:03       3    0,00   11,00   23,00    8,00    0,00    0,00    0,00    0,00   58,00
>>> 12:46:03       4    0,00    0,00   50,00    0,00    0,00    0,00    0,00    0,00   50,00
>>> 12:46:03       5    0,00   13,00   20,00    4,00    0,00    1,00    0,00    0,00   62,00
>>>
>>> So it is only CPU4 that is showing this strange behaviour.
>>>
>>>        
>> Can you adjust irqtop to only count cpu4?  or even just post a few 'cat
>> /proc/interrupts' from that guest.
>>
>> Most likely the timer interrupt for cpu4 died.
>>      
> I've added two keys +/- to your irqtop to focus up and down
> in the row of available CPUs.
> The irqtop for CPU4 shows a constant number of 6 local timer interrupts
> per update, while the other CPUs show various higher values:
>
> irqtop for cpu 4
>
>   eth0                                      188
>   Rescheduling interrupts                   162
>   Local timer interrupts                      6
>   ata_piix                                    3
>   TLB shootdowns                              1
>   Spurious interrupts                         0
>   Machine check exceptions                    0
>
>
> irqtop for cpu 5
>
>   eth0                                      257
>   Local timer interrupts                    251
>   Rescheduling interrupts                   237
>   Spurious interrupts                         0
>   Machine check exceptions                    0
>
> So the timer interrupt for cpu4 is not completely dead but somehow
> broken.

That is incredibly weird.

> What can cause this problem? Any way to speed it up again?
>    

The host has 8 cpus and is only running this 6 vcpu guest, yes?

Can you confirm the other vcpus are ticking at 250 Hz?

What does 'top' show running on cpu 4?  Pressing 'f' 'j' will add a 
last-used-cpu field in the display.

Marcelo, any ideas?

-- 
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