[Xen-devel] weird hvm performance issue.

["Luke S. Crawford" <lsc@xxxxxxxxx>]

(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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