vpxa process consuming 99% CPU time

Hi,

Please help us in providing following information.

We are backing up the VM's on ESX server through Virtual Center. We are using VDDK API "VixDiskLib_Read" to read the data from vmdk file. During this, vpxa process on ESX server is consuming 99% of CPU. And after backup the process releases CPU resource. This is causing ESX server to freeze during backup.

Provide us some information on why vpxa process is consuming 99% of CPU time during backup.

ENVIRONMENT:
=============
Proxy Server: Win2008 R2
VCenter 4.1 update 1

ESX 4.1 Update 1
VDDK 1.2.1

Thanks
Sachin