[Qemu-devel] Behaviour of the qemu-monitor info mem/tlb command

[Daniel Seybold]

Hi everyone,

I've already posted this question to the qemu-user list but I didn't receive 
any answer so maybe you can help
me. I'm not trying to spam the lists ;-)

I'm playing a bit with the qemu-monitor commands, especially with 
the"mem" and "tlb" commands with the intent to get a deeper look into 
theactually memory usage of the guest.

My test setup consists of a host with the following specifications:

CentOS release 6.6 (Final)
Linux version 2.6.32-431.29.2.el6.x86_64
Compiled against library: libvirt 0.10.2
Using library: libvirt 0.10.2
Using API: QEMU 0.10.2
Running hypervisor: QEMU 0.12.1

The guest system is a Ubuntu Server 14.04 (Cloud Image), 4 Cores, 4 GB RAM

On the guest the the load generation tool stress is running andallocates 
100MB of RAM.
Now I'm trying to get the virtual memory mapping inside the guest 
systemwith through this command:

virsh qemu-monitor-command --hmp INSTANCEID 'info mem'

Running this aforementioned setup I observed different behaviours of 
the"info mem"/"info tlb" commands.The follwowing two points describe the 
possible behaviours when runningstress. But I wasn't able to determine 
in which case behaviour 1 or 2occurs.
1. I don't get any results when executing info mem/tlb but there is 
2Gbof RAM allocated inside the VM
2. I get the the virtual memory mapping and physical memory mapping 
fromthe insode of the VM. But if I sum up all the virtual memory pages I 
geta clearly higher amout of memory which is allocated (nearly 1GB) .
AndI get the same results summing up the tlb pages.Or is this the wrong 
approach to get a deeper look into the actualmemory allocation inside a 
VM? (I'm aware of the virsh dommemstatcommand and possible agent based 
solutions inside the VM but I'm tryingto get a the memory information on 
the host level)

Thanks in advance for your help!

Cheers,
Daniel