Windows® Internals, Sixth Edition, Part 2

Finally, by using Process Explorer, you can identify Thread 1 in the IoPriority process by looking at the I/O priority for each of its threads on the Threads tab of its process Properties dialog box. You can also see that the priority for the thread is lower than the default of 8 (normal), which indicates that the thread is probably running in background priority mode. The following screen shot shows what you should expect to see.

Note that if IO Priority sets the priority on File_1 instead of on the issuing thread, both threads would look the same. Only Process Monitor could show you the difference in I/O priorities.

EXPERIMENT: Performance Analysis of I/O Priority Boosting/Bumping

The kernel exposes several internal variables that can be queried through the undocumented SystemLowPriorityIoInformation system class available in NtQuerySystemInformation. However, even without writing or relying on such an application, you can use the local kernel debugger for viewing these numbers on your system. The following variables are available:

IoLowPriorityReadOperationCount and IoLowPriorityWriteOperationCount

IoKernelIssuedIoBoostedCount

IoPagingReadLowPriorityCount and IoPagingWriteLowPriorityCount

IoPagingReadLowPriorityBumpedCount and IoPagingWriteHighPriorityBumpedCount

IoBoostedThreadedIrpCount and IoBoostedPagingIrpCount

IoBlanketBoostCount

You can use the dd memory-dumping command in the kernel debugger to see the values of these variables.

Bandwidth Reservation (Scheduled File I/O)

Windows I/O bandwidth reservation support is useful for applications that desire consistent I/O throughput. Using the SetFileBandwidthReservation call, a media player application asks the I/O system to guarantee it the ability to read data from a device at a specified rate. If the device can deliver data at the requested rate and existing reservations allow it, the I/O system gives the application guidance as to how fast it should issue I/Os and how large the I/Os should be.

The I/O system won’t service other I/Os unless it can satisfy the requirements of applications that have made reservations on the target storage device. Figure 8-27 shows a conceptual timeline of I/Os issued on the same file. The shaded regions are the only ones that will be available to other applications. If I/O bandwidth is already taken, new I/Os will have to wait until the next cycle.

Figure 8-27. Effect of I/O requests during bandwidth reservation

Like the hierarchy prioritization strategy, bandwidth reservation is implemented at the port driver level, which means it is available only for IDE, SATA, or USB-based mass-storage devices.

Container Notifications

Container notifications are specific classes of events that drivers can register for through an asynchronous callback mechanism by using the IoRegisterContainerNotification API and selecting the notification class that interests them. Thus far, one class is implemented in Windows, which is the IoSessionStateNotification class. This class allows drivers to have their registered callback invoked whenever a change in the state of a given session is registered. The following changes are supported:

A session is created or terminated

A user connects to or disconnects from a session

A user logs on to or logs off from a session

By specifying a device object that belongs to a specific session, the driver callback will be active only for that session, while by specifying a global device object (or no device object at all), the driver will receive notifications for all events on a system. This feature is particularly useful for devices that participate in the Plug and Play device redirection functionality that is provided through Terminal Services, which allows a remote device to be visible on the connecting host’s Plug and Play manager bus as well (such as audio or printer device redirection). Once the user disconnects from a session with audio playback, for example, the device driver needs a notification in order to stop redirecting the source audio stream.

Driver Verifier