Windows® Internals, Sixth Edition, Part 2

Internally, these five I/O priorities are divided into two I/O prioritization modes, called strategies. These are the hierarchy prioritization and the idle prioritization strategies. Hierarchy prioritization deals with all the I/O priorities except Very Low. It implements the following strategy:

All critical-priority I/O must be processed before any high-priority I/O.

All high-priority I/O must be processed before any normal-priority I/O.

All normal-priority I/O must be processed before any low-priority I/O.

All low-priority I/O is processed after any higher-priority I/O.

As each application generates I/Os, IRPs are put on different I/O queues based on their priority, and the hierarchy strategy decides the ordering of the operations.

The idle prioritization strategy, on the other hand, uses a separate queue for non-idle priority I/O. Because the system processes all hierarchy prioritized I/O before idle I/O, it’s possible for the I/Os in this queue to be starved, as long as there’s even a single non-idle I/O on the system in the hierarchy priority strategy queue.

To avoid this situation, as well as to control backoff (the sending rate of I/O transfers), the idle strategy uses a timer to monitor the queue and guarantee that at least one I/O is processed per unit of time (typically, half a second). Data written using non-idle I/O priority also causes the cache manager to write modifications to disk immediately instead of doing it later and to bypass its read-ahead logic for read operations that would otherwise preemptively read from the file being accessed. The prioritization strategy also waits for 50 milliseconds after the completion of the last non-idle I/O in order to issue the next idle I/O. Otherwise, idle I/Os would occur in the middle of non-idle streams, causing costly seeks.

Combining these strategies into a virtual global I/O queue for demonstration purposes, a snapshot of this queue might look similar to Figure 8-25. Note that within each queue, the ordering is first-in, first-out (FIFO). The order in the figure is shown only as an example.

Figure 8-25. Sample entries in a global I/O queue

User-mode applications can set I/O priority on three different objects. SetPriorityClass and SetThreadPriority set the priority for all the I/Os that either the entire process or specific threads will generate (the priority is stored in the IRP of each request). SetFileInformationByHandle can set the priority for a specific file object (the priority is stored in the file object). Drivers can also set I/O priority directly on an IRP by using the IoSetIoPriorityHint API.

Note

The I/O priority field in the IRP and/or file object is a hint. There is no guarantee that the I/O priority will be respected or even supported by the different drivers that are part of the storage stack.

The two prioritization strategies are implemented by two different types of drivers. The hierarchy strategy is implemented by the storage port drivers, which are responsible for all I/Os on a specific port, such as ATA, SCSI, or USB. Only the ATA port driver (%SystemRoot%\System32\Ataport.sys) and USB port driver (%SystemRoot%\System32\Usbstor.sys) implement this strategy, while the SCSI and storage port drivers (%SystemRoot%\System32\Scsiport.sys and %SystemRoot%\System32\Stor port.sys) do not.

Note

All port drivers check specifically for Critical priority I/Os and move them ahead of their queues, even if they do not support the full hierarchy mechanism. This mechanism is in place to support critical memory manager paging I/Os to ensure system reliability.

This means that consumer mass storage devices such as IDE or SATA hard drives and USB flash disks will take advantage of I/O prioritization, while devices based on SCSI, Fibre Channel, and iSCSI will not.

On the other hand, it is the system storage class device driver (%SystemRoot%\System32\Class pnp.sys) that enforces the idle strategy, so it automatically applies to I/Os directed at all storage devices, including SCSI drives. This separation ensures that idle I/Os will be subject to back-off algorithms to ensure a reliable system during operation under high idle I/O usage and so that applications that use them can make forward progress. Placing support for this strategy in the Microsoft-provided class driver avoids performance problems that would have been caused by lack of support for it in legacy third-party port drivers.

Figure 8-26 displays a simplified view of the storage stack and where each strategy is implemented. See Chapter 9 for more information on the storage stack.

Figure 8-26. Implementation of I/O prioritization across the storage stack