Arch Linux

pauledd

Member

Registered: 2019-12-14

Posts: 42

Qemu/KVM startup/boot time ram [SOLVED]

Hi

I just spent my arch-desktop 32GB RAM so I can now spend 16GB to my
GPU-Passthrough VM in Qemu/KVM (Windows10,UEFI) (8GB RAM before upgrade...).

All works fine except the time that elapses from the point I press the "Start" button
in virt-manager to the rotating circle of the windows boot screen.

My VM with 4GB RAM took exactly 20s to boot, quite okay...
My VM with 8GB RAM took exactly 30s to boot, still okay...
My VM with 16GB takes now 60s to boot... time for a tea...

Why is that? Is VM-RAM vs. QEMU boot up time a linear correlation?
Do you experience the same boot time behavior?

Last edited by pauledd (2020-07-02 13:07:07)

rowdog

Member

From: East Texas

Registered: 2009-08-19

Posts: 118

Re: Qemu/KVM startup/boot time ram [SOLVED]

I don't use Windows or passthrough but with my Arch VM using virgl, I see no noticeable difference between 4, 8 or 16 GB RAM.

I suspect your RAM upgrade. Maybe you should open the case and double check that the sticks are properly seated and that your old RAM is one bank and the new sticks are in another. Try to isolate if this is related to QEMU or not: start a bunch of other programs and see if things slow down as you pass 16 GB used RAM.

pauledd

Member

Registered: 2019-12-14

Posts: 42

Re: Qemu/KVM startup/boot time ram [SOLVED]

...but with my Arch VM using virgl, I see no noticeable difference between 4, 8 or 16 GB RAM.

Are you using Tianocore / UEFI to boot your VM?

[EDIT]
May be related to this:
https://www.reddit.com/r/VFIO/comments/ … llocation/

Last edited by pauledd (2020-07-01 10:07:25)

rowdog

Member

From: East Texas

Registered: 2009-08-19

Posts: 118

Re: Qemu/KVM startup/boot time ram [SOLVED]

No, I use KVM directly with BIOS booting, no virt-manager or anything like that. I would suspect the ram upgrade not being correct, but I suppose it could be related to how Windows deals with RAM on boot, but I really wouldn't know as I don't do Windows.

Really, all I can say is that, for these amounts of memory, there's no correlation between QEMU and the amount of RAM allocated in terms of slowing the boot process down.

Last edited by rowdog (2020-07-01 17:51:22)

pauledd

Member

Registered: 2019-12-14

Posts: 42

Re: Qemu/KVM startup/boot time ram [SOLVED]

,,,I would suspect the ram upgrade not being correct, but I suppose it could be related to how Windows deals with RAM on boot, but I really wouldn't know as I don't do Windows.

The delay happens before windows takes over the boot process...
I wouldn't suspect a ram upgrade manifesting in a slow VM Boot... If there would be something wrong with my ram it would spit BSOD's in windows and memtest would report some errors which it didn't ... 

I still suspect Tianocore UEFI Firmware allocating ram for the vm taking so long. I'll try to play with hugepages and maybe compiling a custom kernel with that "PREEMPT_NONE=y" option set...

pauledd

Member

Registered: 2019-12-14

Posts: 42

Re: Qemu/KVM startup/boot time ram [SOLVED]

Problem solved. I compiled my own kernel with PREEMPT_NONE=y.
If I now start my VM with 16GB RAM it immediately boots to the Windows boot animation, it was even so fast that I couldn't recognize the Tianocore boot logo

[for reference:  https://www.redhat.com/archives/vfio-us … 00021.html ]

Last edited by pauledd (2020-07-02 13:08:00)