Suatu algoritma penjadwalan CPU kemungkinan melibatkan algoritma yang lain, contohnya algoritma FCFS adalah algoritma RR dengan waktu quantum tertentu. Apa yang dimaksud dengan race condition? Apakah yang dimaksud dengan critical section? Untuk menyelesaikan masalah critical section, ada tiga hal yang harus dipenuhi, sebutkan dan. Pengertian SQL, SQL adalah singkatan dari Structured Query Language. Sedangan pengertian SQL adalah suatu bahasa (language) yang digunakan untuk mengakses data di dalam sebuah database relasional. SQL sering juga disebut dengan istilah query, dan bahasa SQL secara praktiknya digunakan sebagai bahasa standar untuk manajemen database relasional.
In concurrent programming, concurrent accesses to shared resources can lead to unexpected or erroneous behavior, so parts of the program where the shared resource is accessed are protected. This protected section is the critical section or critical region. It cannot be executed by more than one process at a time. Typically, the critical section accesses a shared resource, such as a data structure, a peripheral device, or a network connection, that would not operate correctly in the context of multiple concurrent accesses.[1]
- 3Uses of critical sections
Need for critical sections[edit]
Different codes or processes may consist of the same variable or other resources that need to be read or written but whose results depend on the order in which the actions occur. For example, if a variable ‘x’ is to be read by process A, and process B has to write to the same variable ‘x’ at the same time, process A might get either the old or new value of ‘x’.
Process A:
Process B:
In cases like these, a critical section is important. In the above case, if A needs to read the updated value of ‘x’, executing Process A and Process B at the same time may not give required results. To prevent this, variable ‘x’ is protected by a critical section. First, B gets the access to the section. Once B finishes writing the value, A gets the access to the critical section and variable ‘x’ can be read.
By carefully controlling which variables are modified inside and outside the critical section, concurrent access to the shared variable are prevented. A critical section is typically used when a multi-threaded program must update multiple related variables without a separate thread making conflicting changes to that data. In a related situation, a critical section may be used to ensure that a shared resource, for example, a printer, can only be accessed by one process at a time.
Implementation of critical sections[edit]
The implementation of critical sections vary among different operating systems.
A critical section will usually terminate in finite time,[2] and a thread, task, or process will have to wait for a fixed time to enter it (bounded waiting). To ensure exclusive use of critical sections some synchronization mechanism is required at the entry and exit of the program.
Critical section is a piece of a program that requires mutual exclusion of access.
As shown in Fig 2,[3] in the case of mutual exclusion (Mutex), one thread blocks a critical section by using locking techniques when it needs to access the shared resource and other threads have to wait to get their turn to enter into the section. This prevents conflicts when two or more threads share the same memory space and want to access a common resource.[2]
The simplest method to prevent any change of processor control inside the critical section is implementing a semaphore. In uni processor systems, this can be done by disabling interrupts on entry into the critical section, avoiding system calls that can cause a context switch while inside the section, and restoring interrupts to their previous state on exit. Any thread of execution entering any critical section anywhere in the system will, with this implementation, prevent any other thread, including an interrupt, from being granted processing time on the CPU—and therefore from entering any other critical section or, indeed, any code whatsoever—until the original thread leaves its critical section.
This brute-force approach can be improved upon by using semaphores. To enter a critical section, a thread must obtain a semaphore, which it releases on leaving the section. Other threads are prevented from entering the critical section at the same time as the original thread, but are free to gain control of the CPU and execute other code, including other critical sections that are protected by different semaphores. Semaphore locking also has a time limit to prevent a deadlock condition in which a lock is acquired by a single process for an infinite time stalling the other processes which need to use the shared resource protected by the critical session.
Uses of critical sections[edit]
Kernel-level critical sections[edit]
Typically, critical sections prevent thread and process migration between processors and the preemption of processes and threads by interrupts and other processes and threads.
Critical sections often allow nesting. Nesting allows multiple critical sections to be entered and exited at little cost.
If the scheduler interrupts the current process or thread in a critical section, the scheduler will either allow the currently executing process or thread to run to completion of the critical section, or it will schedule the process or thread for another complete quantum. The scheduler will not migrate the process or thread to another processor, and it will not schedule another process or thread to run while the current process or thread is in a critical section.
Similarly, if an interrupt occurs in a critical section, the interrupt information is recorded for future processing, and execution is returned to the process or thread in the critical section.[4] Once the critical section is exited, and in some cases the scheduled quantum completed, the pending interrupt will be executed. The concept of scheduling quantum applies to 'round-robin' and similar scheduling policies.
![Critical Section Contohnya Adalah Critical Section Contohnya Adalah](https://image.slidesharecdn.com/fkqdtiblrqu4yulc7cv5-140524215617-phpapp02/95/09-sinkronisasi-proses-36-638.jpg?cb=1400968688)
Since critical sections may execute only on the processor on which they are entered, synchronization is only required within the executing processor. This allows critical sections to be entered and exited at almost zero cost. No inter-processor synchronization is required. Only instruction stream synchronization[5] is needed. Most processors provide the required amount of synchronization by the simple act of interrupting the current execution state. This allows critical sections in most cases to be nothing more than a per processor count of critical sections entered.
Performance enhancements include executing pending interrupts at the exit of all critical sections and allowing the scheduler to run at the exit of all critical sections. Furthermore, pending interrupts may be transferred to other processors for execution.
Critical sections should not be used as a long-lasting locking primitive. Critical sections should be kept short enough so that it can be entered, executed, and exited without any interrupts occurring from the hardware and the scheduler.
Kernel-level critical sections are the base of the software lockout issue.
Critical sections in data structures[edit]
In parallel programming, the code is divided into threads. The read-write conflicting variables are split between threads and each thread has a copy of them. Data structures like linked lists, trees, hash tables etc. have data variables that are linked and cannot be split between threads and hence implementing parallelism is very difficult.[6] To improve the efficiency of implementing data structures multiple operations like insertion, deletion, search need to be executed in parallel. While performing these operations, there may be scenarios where the same element is being searched by one thread and is being deleted by another. In such cases, the output may be erroneous. The thread searching the element may have a hit, whereas the other thread may delete it just after that time. These scenarios will cause issues in the program running by providing false data. To prevent this, one method is that the entire>
External links[edit]
- Critical Section documentation on the MSDN Library web page
How many times have you looked at your ballot and thought, “I’d rather not vote for any of these candidates”? It can be quite the dilemma for people who want to exercise their civic duty but tire of having to choose between the lesser of two evils.
This past week, India joined a growing list of countries that permit their citizens to vote without having to choose a candidate. Though the issue has been contested in India, the nation’s Supreme Court ultimately decided it was a critical part of democracy and mandated that voters have the option to choose “none of the above” on their ballots.
In a sense, “none of the above” is a formal vote of protest. It’s a rejection of the current parties and ideologies that dominate the political system. While many who are displeased with all of the candidate options choose not to vote altogether, a lot of time their lack of participation is written off as apathy. By selecting “none of the above,” voters can indicate that they both care and pay attention, but they are not happy with business as usual.
As is the case in most countries with this option, “none of the above” cannot technically win in India. Even if “none of the above” gets the most votes, the position is awarded to the eligible candidate who earns the second highest number of votes. So while this protest won’t ultimately dismantle the political system by seating no one, it at least begins to quantify the discontent of the populace.
Assuming that India’s Supreme Court decision sticks, it joins 11 other countries with a “none of the above” option (or similar variant):
- Colombia
- Ukraine
- Brazil
- Bangladesh
- Finland
- Spain
- Sweden
- Chile
- France
- Belgium
- Greece
Even the United States could join that list so long as an asterisk is affixed. Though it is hardly prevalent throughout the entire country, the state of Nevada has featured a None of the Above option on its ballots since 1975. On one occasion, “none” received the most votes in a Republican primary for Congress. Although the second place vote-getter still received the spot, it sent a clear message that people were uninterested in either candidate.
As great as having a “none of the above” option across the country would be, there has been significant resistance to such efforts. In 2000, California Proposition 23 asked state voters if they’d like to add the option to future ballots, but the measure was handily defeated. Last year, a federal judge tried to ban the practice from persisting in Nevada, though the decision ultimately did not stand.
Nonetheless, “none of the above” could become a valuable tool to more accurately gauge the wishes of voters, particularly in a system seemingly inescapably controlled by two political parties.