[CentOS] Slow HVM IO performance with newer kernels

Hi list.

I am testing CentOS 5.2 now. I experience much slower IO under HVM Guests with the newer kernels, so I started some measurements.

DOM-0 is a fully upgraded CentOS 5.2. DOM-U is CentOS 5.0 not upgraded at all for testing purposes. DOM-U sits on an LVM Volume. No other services are running. Hardware was freshly rebooted every time.

within DOM-0 dd if=/dev/vgpentanol/lvol2 of=/dev/null bs=1M count=1000 gives 134 MB/s

so thats the bare metal value.

within DOM-U dd if=/dev/hda of=dev/null bs=1M count 1000 gives

50 MB/s with DOM-0 kernel 2.6.18-53.1.19.el5xen 10 MB/s with DOM-0 kernel 2.6.18-92.1.1.el5xen 10 MB/s with DOM-0 kernel 2.6.18-92.1.6.el5xen

so there has been a drop to 20% of the performance within 2.6.18-53. Any Ideas? The machine is a test system - so I am open to any changes in configuration you might suggest.

As a side note: A PV Guest gets 85 MB/s no matter which kernel.

Henry