Total Memory Reported in Guest under Memory Balloon

jargon777 · n00b Joined: 12 Mar 2019 Posts: 14

Hi,
This is not really a major issue but basically I observe different behaviour between how my Arch Linux guests and Gentoo guests report the effects of memory ballooning. I am using virt-manager (libvirt/qemu) to manage my guests. In Gentoo, meminfo's MemTotal does not change when the allocated memory to a guest is changed, but the amount of memory in use (MemFree) does. In the Arch Linux guest, the MemTotal amount changes while the MemFree amount doesn't.

In both cases, however, the host still receives the memory back (hence why this is minor). I suspect that I am missing a kernel option or something and the Gentoo guest is not re-mapping the user space memory when the balloon changes the memory?

jargon777 · n00b Joined: 12 Mar 2019 Posts: 14

I guess no one has any insights, I suppose then instead it would be nice to get some pointers so I can try and debug and compare what the difference is between the two. It's mostly out of curiosity at this point, I have observed this difference for some time and haven't been able to find a way to understand why it's different. Mostly the annoyance is visual in that my htop memory bar in Gentoo is consumed with "in use memory".

Things I have tried:
- Triggering the balloon manually to try and capture events in the Arch and Gentoo. Haven't actually been able to capture anything though.
- Trying to identify what consumes the memory. In top/htop it's just marked as 'used', but there's no way to identify what process or thing is using the memory. Presumably it's the virtio driver.
- Trying to use the install disks to compare behaviour. The archlinux install CD also resizes but the gentoo install disk doesn't.

mike155 · Posted: Sun May 09, 2021 9:01 am Post subject:

Your problem looks interesting. Let's try to find out what is going on.

Please show us the command line arguments that are used to start your guest machines on your host machine. Run

Code:

ps -fe | grep qemu

and post the output. You may want to remove private data if there's anything you don't want to post.
Please post the output of

Code:

zgrep -E "(VIRTIO|KVM)" /proc/config.gz

in your two guest machines.
Please post the output of

Code:

uname -a

on your host machine as well as on your two guest machines.

jargon777 · n00b Joined: 12 Mar 2019 Posts: 14

vpn-gate is the archlinux guest, and gitea is the gentoo guest

mike155 · Posted: Wed May 12, 2021 4:42 pm Post subject:

There are differences in the way your virtual machines are started:

Code:

-machine pc-q35-5.2
-machine pc-i440fx-3.1

-cpu SandyBridge,vme=on,ss=on,vmx=on,pdcm=on,pcid=on,hypervisor=on,arat=on,tsc-adjust=on,umip=on,arch-capabilities=on,xsaveopt=on,pdpe1gb=on,skip-l1dfl-vmentry=on,pschange-mc-no=on
-cpu host,migratable=on

-m 1024
-m 4096

-smp 1,sockets=1,cores=1,threads=1
-smp 2,sockets=2,cores=1,threads=1

-device virtio-balloon-pci,id=balloon0,bus=pci.4,addr=0x0
-device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6,deflate-on-oom=on

Make sure that the two VMs are started in exactly the same way and with the same QEMU/KVM parameters.

There are many differences in the kernel config parameters of the two guest machines. Change the kernel config of one of your guest machines so that it matches the config of the other VM.