The IBM System/370 (S/370) family of mainframe computers was introduced in 1970 and was used widely in large organizations for several decades. One of the key features of the S/370 was its ability to support multiple virtual machines running on a single physical system. This allowed multiple users and applications to share the same hardware resources, including CPU time, memory, and I/O.
In order to manage the sharing of system resources among different users and applications, IBM developed a system of "shares" that could be allocated to different tasks based on their priority and resource requirements. The concept of shares is similar to the concept of "slices" used in other time-sharing systems.
Shares were allocated to different resources based on the following factors:
CPU share: The amount of CPU time allocated to a task was based on the number of CPU shares assigned to that task. The higher the number of shares, the more CPU time the task would receive.
Memory share: The amount of memory allocated to a task was based on the number of memory shares assigned to that task. The higher the number of shares, the more memory the task would receive.
I/O share: The amount of I/O bandwidth allocated to a task was based on the number of I/O shares assigned to that task. The higher the number of shares, the more I/O bandwidth the task would receive.
By allocating shares to different tasks, the S/370 was able to provide fair and efficient sharing of system resources among multiple users and applications. Shares could be dynamically adjusted based on changing resource requirements, allowing the system to adapt to changing workload demands.