Re: slow guest performance with build load, looking for ideas

[Avi Kivity <avi@redhat.com>]

On 06/28/2009 10:05 PM, Erik Jacobson wrote:
>>>    * In some of the timing runs on this system, the "real time" reported by
>>>      the time command was off by 10 to 11 times.  Issues were found in
>>>      the messages file that seemed to relate to this including HUGE time
>>>      adjustments by NTP and kernel hrtimer 'interrupt too slow' messages.
>>>      This specific problem seems to be intermittent.
>>>        
>> This is on the host? It can easily ruin your day.
>>      
>
> This was in the guest.
>    

Ok.  Please keep ntp off in the guest and verify the guest says it uses 
kvmclock.


>> Can you post kvm_stat output during the run?
>>      
>
> Sure, I'll try to get time on the system again next week and post in
> to the thread again.  We'll still have the issue with the non-sequential
> nodes and incorrect representation of memory for this two-socket
> Nehalem system.  I don't think that patch has made it in to the kernel.
>
> Thanks for replying back.
>
> If you have any other things you'd suggest trying, I'm game to give it a
> whirl.  Someone suggested trying to export a whole PCI device to the guest.
> I won't be able to do that on this machine, maybe later when I have physical
> access to the system.  Besides, that exercise might not poke at what I'm
> interested in anyway.
>    


Shouldn't be needed.  It's sufficient to export an LVM volume with 
cache=none:

    -drive file=/dev/vg/lv,cache=none,if=virtio

> Others suggested some potential settings EPT (Extended Page Table) and
> VPID (Virtual Path Identifier?) but I don't see where these settings are
> exposed (they aren't, for example, in this system's BIOS).
>    

Look in /sys/modules/kvm_intel/paramters, ept and vpid should default to 
enabled.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.