The Adventures of a Systems Engineer

By default the Xen Hypervisor uses the credit CPU scheduler. The sEDF (simple earliest deadline first) CPU scheduler can be used by setting the sched boot parameter equal to sedf. The easiest way to do this is by editing the grub boot loader config file, menu.lst. The file is located in /boot/grub/menu.lst. The file must be edited by a root user (this can be done in Ubuntu by using the sudo command). This is demonstrated in the entry below. I just copied the existing Xen boot entry and added this one with the sched parameter so I could choose between schedulers at boot time.

title    Xen 3.3 / Ubuntu 8.04.3 LTS, kernel 2.6.24-26-xen sEDF SCHEDULER

root     (hd1,0)

kernel   /boot/xen-3.3.gz sched=sedf

module   /boot/vmlinuz-2.6.24-26-xen root=UUID=e8c13945-5158 console=tty0

module   /boot/initrd.img-2.6.24-26-xen

quiet

I have been setting up a test Xen 3.3 box running on Ubuntu 8.04. I have been getting the following error: “Device xxxx (vbd) cannot be connected. failed to find an unused loop device”  where xxxx is some number. It turns out that by default only 8 loopback devices are enabled. The following command shows how many loopback devices are enabled:

ls -l /dev/loop*

To increase the number of loopback devices the following line needs to be added to the /etc/modules file. I used a value of 64 so I can run up to 32 virtual machines.

add loop max_loop=64

Now when you run ls -l /dev/loop* you will see additional loopback devices.