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[Xen-devel] weird hvm performance issue.


  • To: xen-devel@xxxxxxxxxxxxxxxxxxx
  • From: "Luke S. Crawford" <lsc@xxxxxxxxx>
  • Date: Mon, 29 Jan 2007 10:08:52 -0800 (PST)
  • Delivery-date: Mon, 29 Jan 2007 10:08:32 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>


(I posted this on xen-users last week, and nobody seemed to know what was up. It is time, I suppose then, to dust off the debugger; I was hoping one of you could point me in the correct direction to begin troubleshooting this.)

I'm HVM booting our install-disks (systemimager) here, and I'm spotting some weird performance issues. Our install uses rsync and prints every file as it copies it. I notice that files scroll quickly for a while, then slowly. the slowness continues until I press a button (within xm console) at which point the slowness immediately vanishes.


I watch 'top' in the Dom0 in another window, and I see that when files scroll slowly, qemu-dm uses at most 1% cpu; once I press a key and it moves fast again, qemu-dm uses up to 70% of a CPU. (this is on a box with 4 cores) The correlation is extremely strong; I'm fairly certain it's not just in my head.

The CPU usage of qemu-dm correlates with the speed of the scroll; exiting from the xm console does not appear to effect the cpu usage of qemu-dm- it will drop to 1% after several minutes.

any ideas what is going on here? any ideas how I could debug this further?

my xm config file looks something like this:

kernel = "/usr/lib/xen/boot/hvmloader"
device_model = '/usr/lib64/xen/bin/qemu-dm'
builder='hvm'
memory = 512
shadow_memory = 8
vcpus=1
vif = [ 'type=ioemu, bridge=xenbr0' ]
nographic=1
sdl=0
vnc=0
serial='pty'
name = "VM1-64"
fda='/var/tmp/disk_dev.img'
disk = ['file:/var/tmp/si.iso,hdb:cdrom,w',
'phy:/dev/sda5,hda,w']
boot='da'
on_poweroff = 'destroy'
on_crash    = 'restart'



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