time command in vm

time command in vm

[Terry]

Hi All,

When we use time command in vm, we can get 'elapsed time', 'user time' 
and 'system time'.  How to explain these three times in detail?  For 
example, when we have a shadow page fault, we exit from guest to host 
for handling the fault.  So, this handling time should be considered in??

Thanks,
Terry

Re: time command in vm

[David S. Ahern]

Terry wrote:
> Hi All,
> 
> When we use time command in vm, we can get 'elapsed time', 'user time'
> and 'system time'.  How to explain these three times in detail?  For
> example, when we have a shadow page fault, we exit from guest to host
> for handling the fault.  So, this handling time should be considered in??
> 

I believe the time spent within kvm handling faults and such for the
guest shows up as system time to the guest.

david


> Thanks,
> Terry
> 
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Re: time command in vm

[Christian Borntraeger]

Am Dienstag, 2. September 2008 schrieb David S. Ahern:
> 
> Terry wrote:
> > Hi All,
> > 
> > When we use time command in vm, we can get 'elapsed time', 'user time'
> > and 'system time'.  How to explain these three times in detail?  For
> > example, when we have a shadow page fault, we exit from guest to host
> > for handling the fault.  So, this handling time should be considered in??
> > 
> 
> I believe the time spent within kvm handling faults and such for the
> guest shows up as system time to the guest.
> 
> david

It depends on the time accounting.

Systems with CONFIG_VIRT_CPU_ACCOUNTING (s390 and ppc64) will account host 
page fault handling as steal time in the guest. guest user and guest system 
time remain unchanged. (Host system time will increase of course)

On other systems (e.g. x86), I think the host page fault time will be 
accounted to either guest system or guest user, depending on where the next 
tick triggers accouting. 

elapsed time will increase on both kind of system.

Christian

Re: time command in vm

[Avi Kivity]

David S. Ahern wrote:
> Terry wrote:
>   
>> Hi All,
>>
>> When we use time command in vm, we can get 'elapsed time', 'user time'
>> and 'system time'.  How to explain these three times in detail?  For
>> example, when we have a shadow page fault, we exit from guest to host
>> for handling the fault.  So, this handling time should be considered in??
>>
>>     
>
> I believe the time spent within kvm handling faults and such for the
> guest shows up as system time to the guest.
>   

It depends.  If the fault happens in guest userspace, it would be 
accounted as guest user time.  From the guest's point of view, it's just 
a slow memory access.

-- 
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