guest threads

guest threads

[tarski]

hello everyone,
    
     I was just wondering how KVM schedules multiple Virtual machines (guest
operating systems) running on top of it? If a thread/process blocks on a guest
os does KVM block the whole virtual machine or are other threads/processes
running on that particular guest given a chance to run?

Thanks

Tarski


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Re: guest threads

[Dor Laor]

>
>hello everyone,
>
>     I was just wondering how KVM schedules multiple Virtual machines
>(guest
>operating systems) running on top of it? If a thread/process blocks on
a
>guest
>os does KVM block the whole virtual machine or are other
threads/processes
>running on that particular guest given a chance to run?

The KVM does not schedule processes at all. Actually this is its power
over Xen. There is no need to build a complex schedule, we're using such
- the Linux kernel.

Currently, except for aio, there are no threads in qemu so our user
space does not use threads. Had it used threads and for the aio case,
the execution of the guest relays on the call to ioctl kvm_run.
So without aio, the guest is not running while waiting for disk.
With aio the guest is running in the main thread context while another
thread serves the aio.

>
>Thanks
>
>Tarski
>
>
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Re: guest threads

[Avi Kivity]

tarski wrote:
> hello everyone,
>     
>      I was just wondering how KVM schedules multiple Virtual machines (guest
> operating systems) running on top of it? If a thread/process blocks on a guest
> os does KVM block the whole virtual machine or are other threads/processes
> running on that particular guest given a chance to run?
>
>   
kvm is not aware of threads and processes within guests.

kvm (actually, the Linux scheduler) will deschedule a guest if one of 
these two conditions occur:

- if the guest executes the 'hlt' instruction (typically when it has no 
processes to run)
- using normal Linux process preemption rules (timeslice expires, higher 
priority thread becomes runnable, etc.)

so it's quite possible for multiple guest threads or processes to run 
within a single host timeslice.

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


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