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Device Allocation Techniques

In the operating system, device allocation and recycling are important functions of device management, which aims to optimize resource usage, avoid conflicts, and improve system efficiency. The following are the main allocation and recycling strategies:

Allocation of exclusive devices

  • Static allocation:
  • Before the job starts executing, the system allocates all devices, controllers, and channels required for the job at one time.
  • Advantages: There will be no deadlock problem.
  • Disadvantages: Low device utilization because resources are exclusively used during the entire job and cannot be released even when the job is no longer needed.
  • Dynamic allocation:
  • Allocate devices on demand during process execution. When a process needs to use a device, it requests allocation through a system call and releases it immediately after use.
  • Advantages: Improve device utilization.
  • Disadvantages: If the allocation strategy is not appropriate, it may cause process deadlock.

Data structure of device allocation

In order to manage device allocation, the system uses the following control tables:

  • Device Control Table (DCT):
  • Record basic information about the device, including device type, status (such as allocated, unallocated, etc.) and device queue, etc.
  • Controller Control Table (COCT):
  • Records the usage status and connection status of the controller.
  • Channel Control Table (CHCT):
  • Manages the usage status of the channel.
  • System Device Table (SDT):
  • Records the basic information of all devices in the system, such as device type, quantity, driver address, etc.
  • Logical Unit Table (LUT):
  • Maps logical device names to physical device names to achieve device independence and allow users to request devices through logical names.

Device Recycling

  • When the device is used up, the system modifies the device to "unallocated" status according to the device status table, and selects the next process that needs to use the device from the waiting queue for allocation.
  • For shared devices, the system releases the device after each I/O operation is completed, allowing other waiting processes to use the device in time.
  • Use a common buffer pool structure to optimize resource utilization, avoid long-term idleness of devices, and improve the overall efficiency of the system.

Device independence

  • Device independence separates applications from specific physical devices. Users only need to use logical device names and do not care about the actual allocated physical devices.
  • Device drivers and device independence software manage all devices through a unified interface, perform device allocation and recycling, and improve the scalability and adaptability of the system.