[vfio-users] VM slows down with +2gb RAM

[Alex Williamson]

On Mon, 2015-10-19 at 13:01 +0200, Manuel Zeusin wrote:
> Hello,
> 
> I try to make a multi-os gaming machine, one win 10 with NVIDIA 970 and
> other with win10 and AMD 7770... I can do it work and works like a
> charm...but with 2gb...
> 
> My host is a 5820k with 4*4gb RAM, gigabyte X99-ud4 ( a shitty mobo ),
> radeon 2600hd for host, NVIDIA 970 and radeon 7770.
> 
> For example, i do a fresh installation of windows for NVIDIA, with 2gb RAM.
> The installation complete and GPU have vídeo, shutdown the computer and
> assign 4gb, Windows boot and everything slows down at almost freeze and CPU
> goes to 80-100% with no reason, it doesnt matter if have 1vcpu or 10vcpu.
> gpu sometimes have vídeo and sometimes not...  But.... If i remove VGA PCI
> assignement, windows boot fine and works like a charm.

What's happening on the host when this slowdown occurs?  If the host is
under memory pressure and spending time swapping, then it's going to
affect the guest performance.  Have you tried using hugepages?  If you
want to test whether the slowdown is imposed by the VM or the host,
reserved 4GB worth of hugepages, run the VM with 2G, then shutdown and
restart the VM with 4G.  The host memory pressure should be the same in
both cases since the same hugepages are reserved for both.  Also take
into account that Windows is going to size its swap based on the VM
memory size and it may take some time to re-allocate things if you've
just changed the memory size.  I run a 4G GPU VM regularly with quite
good performance and I'm sure the typical user here provides more than
2G to their VM as well.  Thanks,

Alex