The other kernel module we will discuss is RSIoCnt. Its job is to count the pending synchronous I/O's and store this count in shared memory where the idleness detector can access it. It must also tell RSTask to resume any paused schedule whenever this count becomes zero.
We implemented RSIoCnt as a file system filter driver. A filter driver implements a filter device, a special kind of device extremely helpful in tracing system events in Windows NT/2000. A filter device can attach to an existing device, causing it to intercept any requests destined for that existing device. For more information about them, see [15,10]. Our filter driver has low overhead because it merely counts the requests as they start and stop and passes them on.
Unfortunately, our approach limits one to filtering only non-network file systems. There are undocumented ways to filter network file systems and network devices, as shown in [10], but we do not do this in our prototype due to our stability goal.