• • The currently running process may be interrupted and moved to the ready state by the OS.
  • The resources (mainly CPU cycles) are allocated to the process for the limited amount of time and then is taken away, and the process is again placed back in the ready queue if that process still has CPU burst time remaining.

• • That process stays in ready queue till it gets next chance to execute.
  • The decision to preempt may be performed when,
    • A new process arrives (with a higher priority than the current process)

Preemptive Scheduling

• • An interrupt occurs (fairness)
  • Periodically based on clock interrupt. (Eg: Round Robin)

• More robust, one process cannot monopolize the CPU.
• Fairness. The OS makes sure that CPU usage is the same by all running process.

• Context switch cost: Time taken by the scheduler to suspend the running task, switch the context, and dispatch the new incoming task.

• Memory management is the process of controlling and coordinating computer memory, assigning portions called blocks to various running programs to optimize overall system performance.
• Memory management resides in hardware, in the OS (operating system), and in applications.
• In hardware, memory management involves components that physically store data, such as RAM chips, memory caches,etc.,.
• In the OS, memory management involves the allocation (and constant reallocation) of specific memory blocks to individual programs as user demands change.
• At the application level, memory management ensures the availability of adequate memory for the objects and data structures of each running program at all times.Application memory management combines two related tasks, known as allocation and recycling.
  • Allocation : When the program requests a block of memory, a part of the memory manager called the allocator assigns that block to the program.
  • Recycling : When a program no longer needs the data in previously allocated memory blocks, those blocks become available for reassignment. This task can be done manually (by the programmer) or automatically (by the memory manager).

• As processes are loaded and removed from memory, the free memory space has broken into small pieces.
• That processes cannot be allocated to memory blocks considering their small size and memory blocks remains unused.
• This problem is known as Fragmentation.There are two types of fragmentation,
  • Internal fragmentation
  • External fragmentation

• Memory block assigned to process is lager. Memory is left unused in some portion as it cannot be used by another process.

• Total memory space is enough to satisfy a request or to reside a process in it, but it is not contiguous, so it cannot be used.

• A thread is a flow of execution through the process code, with its own program counter that keeps way of which instruction to execute next, system registers which hold its existing working variables, and a stack which contains the execution history. A thread is also called as lightweight process.
• A thread shares with its peer threads few information like code segment, data segment and open files.
• To improve application performance through parallelism.
• Threads represent a software approach to improving performance of operating system by reducing the overhead thread is equivalent to a classical process.
• Each thread has exactly one process and no thread can exist outside a process. Each thread signifies a separate flow of control.
• Implementing network servers and web server.
• They are also offer a suitable foundation for parallel execution of applications on shared memory multiprocessors.
• The working process of a single-threaded and a multithreaded are shows in the following figure.

• The context switching time minimize in thread concepts.
• Use of threads offers concurrency within a process.
• Efficient communication.
• It is more economical to create and context switch threads.
• Threads allow utilization to a greater scale and efficiency of multiprocessor architectures.

• There are two types of thread,
  • User Level Threads.
  • Kernel Level Threads.

• In this case, the thread management kernel is not aware of the existence of threads.
• The thread library contains code for creating and destroying threads, for transient message and data between threads, for scheduling thread execution and for saving and reestablishing thread settings.
• From the single thread start the application.

• Kernel is done thread management.
• In the application area there is no thread management code.
• The operating system directly supported Kernel threads.
• Multithreaded can be programmed in an any application.
• An application is supported within a single process.

• Scheduling is an essential part of a Multiprogramming operating systems.
• Such operating systems allow more than one process to be loaded into the executable memory at a given time and the loaded process shares from the CPU using time multiplexing.
• The Operating System maintains the following important process scheduling queues −
  • Job queue − This queue preserves all the processes in the system.
  • Ready queue − This queue keeps a set of all processes residing in main memory, ready and waiting to execute. A new process is always placed in this queue.
  • Device queues − The processes which are blocked due to inaccessibility of an I/O device establish this queue.

• The OS can use different procedures to manage each queue (FIFO, Round Robin, Priority, etc.).
• The OS scheduler determines how to move from processes between the ready and run queues which can only have one entry per processor core on the system; in the above diagram, it has been combined with the CPU.

• Long-Term Scheduler
• Short-Term Scheduler
• Medium-Term Scheduler

• It is also called a job scheduler.
• It selects processes from the queue and loads into memory for execution.
• The long-term scheduler may not be accessible or minimal.
• No long-term scheduler in the time-sharing operating systems.
• Use of long-term scheduler when a process changes the state from new to ready.

• Short term scheduler is also known as CPU scheduler.
• It is used to improve system performance.
• It is change from ready state to running state of the process.
• Short term scheduler is faster than compared to long term scheduler.

• Medium-term scheduling is a part of swapping.
• Medium Term Scheduler has removed the processes from the memory.
• To remove the process from memory and make space to the other processes, the postponed process is moved to the secondary storage.
• This process is called swapping.

• Caching is an area of a computer’s memory devoted to temporarily storing recently used data.
• In caching the content are which includes HTML pages, images, files and Web objects, is stored on the local hard drive in order to make it faster for the user to access it, which helps to develop the efficiency of the computer and its overall performance.
• For example, when a user returns to a Web page they have recently accessed, the browser can pull those files from the cache instead of the original server because it has stored the user’s activity.
• The storing of that information saves the user can getting to it faster, and less time to the traffic on the network.

• When a cache client wants to access data, it first checks the cache. Cache hit is known as when the requested data is found.
• The current of attempts that result in cache hits is known as the cache hit rate or ratio.
• If the requested data isn’t found in the cache a situation known as a cache miss. It is getting from main memory and through into the cache. How this is done, and what data is ejected from the cache to make room for the new data, depends on the caching algorithm or policies the system uses.

• Web browsers, such as Internet Explorer, Firefox, Safari and Chrome, use a browser cache to develop performance of frequently accessed webpages.
• When we visit a webpage, the requested files are stored in computing storage in the browser’s cache.
• Clicking back and returning to a previous page enables from the browser to retrieve most of the files it needs from the cache instead of having them all resent from the web server.
• This approach is called read cache. The browser can read data from the browser cache much faster than from the webpage it can reread the files.

• Some examples of cache algorithms they are,
  • Least Frequently Used (LFU)
  • Least Recently Used (LRU)
  • Most Recently Used (MRU)

• Keeps track of how often an entry is accessed. The data that has to use lowest count gets then removed first.

• Puts recently accessed data to near the top of the cache. When the cache reaches its limit, the least recently accessed data are removed.

• Removes the most recently accessed data first. This approach is best when older items are more likely to be used.

• Deadlock is one of the part of multithreading.
• synchronized keyword is used to make the class or method thread-safe which means only one thread can have lock of synchronized method and use it, other threads have to wait till the lock releases and anyone of them acquire that lock.
• Deadlock can occur in a situation when a thread is waiting for an object lock, that is acquired by another thread and second thread is waiting for an object lock that is acquired by first thread.
• Since, both threads are waiting for each other to release the lock, the condition is called deadlock.