Re: real mode emulation and memory consumption

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Kamble, Nitin A wrote:
>> Kamble, Nitin A wrote:
>>     
>>> Hi Avi,
>>>  I am noticing that with SL 10.1 The QEMU process memory consumption
>>> steadily increases, up the the guest memory size and then the guest
>>>       
> dies
>   
>>> with unhandled vmexit. If I change the guest memory size I can see
>>>       
> the
>   
>>> qemu process dies accordingly, after reaching the size of allotted
>>>       
> guest
>   
>>> memory.
>>>   Does this hint you to any issues?
>>>
>>>
>>>       
>> Are you talking about RSS?  That's normal behavior (except for the
>> untimely death).  kvm allocates memory on demand, like regular Linux
>> processes.
>>
>>     
>
> Yes, the RES column in the TOP. Why is it normal behavior? 

When the guest accesses a page for the first time, it is added to RSS. 
When Linux swaps it out, it is removed from RSS.

> I see the
> guest is looping in the boot-loader switching between real mode and
> protected mode without make any progress, and at the same time the
> residential memory consumption of the qemu process is steadily
> increasing, and once it crosses the amount of physical memory configured
> for the guest, the KVM driver gets the invalid vmexit.
>   

Well, this part is not normal ;)  however I have no insight as to why it
is happening.

>   The interesting part is if I change the amount of physical memory to
> the guest, the time when the guest dies changes accordingly, and looking
> at the top output one can predict when it is going to crash.
>   Where is that memory getting consumed in the qemu process? Why is it
> affecting the KVM driver?

It's probably just a bug with a surprising side effect.  Maybe it is a
memory-clearing loop gone wild.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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