Fresh installation of Debian Jessie 8.5, or in my case from a Turnkeylinux Foodsoft ISO, as a guest system on MS Hyper-V host.
It might run fine for some hours, but it keeps getting slower. „top“ command shows ksoftirqd/X (0, 1, …) causing high CPU load.
Solution / Workaround
Edit /etc/default/grub and add parameter „nohz=off“.
After saving, don’t forget to run „update-grub“ and reboot.
Also disable dynamic memory allocation (ballooning), it seems to be not fully supported yet.
If you do not want to change grub boot options for some reason, or the issue does not get solved, you can also use a backport kernel (i.e. a newer one). Current default version 3.16 seems to have problems with virtualization, so this might help.
Follow these steps (create a snapshot before!):
echo "#Backports" >> /etc/apt/sources.list && echo deb http://ftp.de.debian.org/debian/ jessie-backports main >> /etc/apt/sources.list
apt-get -t jessie-backports install linux-image-amd64
#afterwards, test it with: uname -r