第6章インフラストラクチャーの抽象化層：オペレーティングシステム

The ability to run multiple programs at once on computers today has a huge productivity impact in all industries. In concurrent programming, managing shared resources among multiple threads or processes is crucial for maintaining data integrity and preventing race conditions. One common scenario is the producer-consumer problem, where one or more threads (producers) generate data, and others (consumers) consume it. To avoid conflicts, synchronization mechanisms like locks and condition variables are employed. Locks help ensure that only one thread accesses a critical section of code at a time. In the context of the producer-consumer problem, locks can be utilized to safeguard shared data structures, preventing simultaneous access by multiple threads and ensuring data consistency.

Condition variables are another synchronization tool that allows threads to coordinate their activities. In the context of producers and consumers, a condition variable could signal when data is available for consumption or when space is available for production. Threads can use these signals to efficiently wait for or notify others about the state of shared resources. The combination of locks and condition variables provides a powerful means to synchronize complex interactions between producers and consumers, ensuring orderly access to shared resources.

Despite the benefits of synchronization mechanisms, the improper use of locks can lead to issues like deadlocks, where two or more threads or processes are stuck in a circular wait, unable to proceed because each is waiting for the other to release a resource. This situation can bring a system to a standstill, and careful design and coding practices are necessary to prevent or detect and recover from deadlocks. To handle deadlocks, techniques such as deadlock detection algorithms and prevention strategies are employed, contributing to the robustness of concurrent systems.

The method that defines how data is stored in the memory by providing a set of requests for resources and identifying which processes should be given which resources to make the most efficient use of the resources is called allocation. Like scheduling, allocation is another kind of decision-making that an OS performs about how to use resources to support concurrency. There are three main forms of allocation: contiguous allocation, linked allocation, and indexed allocation.

In contiguous allocation, each file is assigned to a contiguous (i.e., neighboring) set of blocks in the memory. For example, if a file requires three blocks and is given a starting location x, the file will be allocated in x, x + 1, x + 2. The directory entry with contiguous allocation contains the starting block address and the length of the file. In linked allocation, each file is a linked list of memory blocks. Using the same example, the first block will be allocated in location x and it will include the address of the second block. The directory entry with linked allocation contains a pointer to the starting block and a pointer to the last block. In indexed allocation, each file has an index block containing the pointers to all blocks for that file.