CoolInterview.com - World's Largest Collection of Interview Questions & Answers, FAQs, queries, sample papers, exam papers, dumps, what, why, how, where, when questions
Our Services
Get 9,000 Interview Questions & Answers in an eBook.




Get it now !!
Send your Resume to 6000 Companies

Search Interview Questions

Question: What is page thrashing?

Answer: Some operating systems (such as UNIX or Windows in enhanced mode) use virtual memory. Virtual
memory is a technique for making a machine behave as if it had more memory than it really has, by using disk space to simulate RAM (random-access memory). In the 80386 and higher Intel CPU chips, and in most other modern microprocessors (such as the Motorola 68030, Sparc, and Power PC), exists a piece of hardware called the Memory Management Unit, or MMU.

The MMU treats memory as if it were composed of a series of ?pages.? A page of memory is a block of
contiguous bytes of a certain size, usually 4096 or 8192 bytes. The operating system sets up and maintains a table for each running program called the Process Memory Map, or PMM. This is a table of all the pages of memory that program can access and where each is really located.

Every time your program accesses any portion of memory, the address (called a ?virtual address?) is processed by the MMU. The MMU looks in the PMM to find out where the memory is really located (called the ?physical address?). The physical address can be any location in memory or on disk that the operating system has assigned for it. If the location the program wants to access is on disk, the page containing it must be read from disk into memory, and the PMM must be updated to reflect this action (this is called a ?page fault?).

Because accessing the disk is so much slower than accessing RAM, the operating system tries to keep as much of the virtual memory as possible in RAM. If you?re running a large enough program (or several small programs at once), there might not be enough RAM to hold all the memory used by the programs, so some of it must be moved out of RAM and onto disk (this action is called ?paging out?).
The operating system tries to guess which areas of memory aren?t likely to be used for a while (usually based on how the memory has been used in the past). If it guesses wrong, or if your programs are accessing lots of memory in lots of places, many page faults will occur in order to read in the pages that were paged out. Because all of RAM is being used, for each page read in to be accessed, another page must be paged out. This can lead to more page faults, because now a different page of memory has been moved to disk.

The problem of many page faults occurring in a short time, called ?page thrashing,? can drastically cut the performance of a system. Programs that frequently access many widely separated locations in memory are more likely to cause page thrashing on a system. So is running many small programs that all continue to run even when you are not actively using them. To reduce page thrashing, you can run fewer programs simultaneously. Or you can try changing the way a large program works to maximize the capability of the operating system to guess which pages won?t be needed. You can achieve this effect by caching values or changing lookup algorithms in large data structures, or sometimes by changing to a memory allocation library which provides an implementation of malloc() that allocates memory more efficiently. Finally, you might consider adding more RAM to the system to reduce the need to page out.



Category C Interview Questions & Answers - Exam Mode / Learning Mode
Rating (3.7) By 75 users
Added on 10/22/2009
Views 9134
Rate it!
Question: What is page thrashing?
Answer:

Some operating systems (such as UNIX or Windows in enhanced mode) use virtual memory. Virtual
memory is a technique for making a machine behave as if it had more memory than it really has, by using disk space to simulate RAM (random-access memory). In the 80386 and higher Intel CPU chips, and in most other modern microprocessors (such as the Motorola 68030, Sparc, and Power PC), exists a piece of hardware called the Memory Management Unit, or MMU.

The MMU treats memory as if it were composed of a series of ?pages.? A page of memory is a block of
contiguous bytes of a certain size, usually 4096 or 8192 bytes. The operating system sets up and maintains a table for each running program called the Process Memory Map, or PMM. This is a table of all the pages of memory that program can access and where each is really located.

Every time your program accesses any portion of memory, the address (called a ?virtual address?) is processed by the MMU. The MMU looks in the PMM to find out where the memory is really located (called the ?physical address?). The physical address can be any location in memory or on disk that the operating system has assigned for it. If the location the program wants to access is on disk, the page containing it must be read from disk into memory, and the PMM must be updated to reflect this action (this is called a ?page fault?).

Because accessing the disk is so much slower than accessing RAM, the operating system tries to keep as much of the virtual memory as possible in RAM. If you?re running a large enough program (or several small programs at once), there might not be enough RAM to hold all the memory used by the programs, so some of it must be moved out of RAM and onto disk (this action is called ?paging out?).
The operating system tries to guess which areas of memory aren?t likely to be used for a while (usually based on how the memory has been used in the past). If it guesses wrong, or if your programs are accessing lots of memory in lots of places, many page faults will occur in order to read in the pages that were paged out. Because all of RAM is being used, for each page read in to be accessed, another page must be paged out. This can lead to more page faults, because now a different page of memory has been moved to disk.

The problem of many page faults occurring in a short time, called ?page thrashing,? can drastically cut the performance of a system. Programs that frequently access many widely separated locations in memory are more likely to cause page thrashing on a system. So is running many small programs that all continue to run even when you are not actively using them. To reduce page thrashing, you can run fewer programs simultaneously. Or you can try changing the way a large program works to maximize the capability of the operating system to guess which pages won?t be needed. You can achieve this effect by caching values or changing lookup algorithms in large data structures, or sometimes by changing to a memory allocation library which provides an implementation of malloc() that allocates memory more efficiently. Finally, you might consider adding more RAM to the system to reduce the need to page out.
Source: CoolInterview.com


thrashing is a situation where large amounts of computer resources are used to do a minimal amount of work, with the system in a continual state of resource contention Source: CoolInterview.com

Answered by: anil kumar | Date: 7/15/2010 | Contact anil kumar Contact anil kumar


If you have the better answer, then send it to us. We will display your answer after the approval

Rules to Post Answers in CoolInterview.com:-

  • There should not be any Spelling Mistakes.
  • There should not be any Gramatical Errors.
  • Answers must not contain any bad words.
  • Answers should not be the repeat of same answer, already approved.
  • Answer should be complete in itself.

Post your answer here

Inform me about updated answers to this question.
Related Questions
View Answer
When should the register modifier be used? Does it really help?
View Answer
When should a type cast be used?
View Answer

Please Note: We keep on updating better answers to this site. In case you are looking for Jobs, Pls Click Here Vyoms.com - Best Freshers & Experienced Jobs Website.

View All C Interview Questions & Answers - Exam Mode / Learning Mode




India News Network
Latest 20 Questions
An offer which is open for acceptance over a period of time is: (a) Cross Offer (b) Counter Offer (c) Standing Offer (d) Implied Offer
Specific offer can be communicated to__________ (a) All the parties of contract (b) General public in universe (c) Specific person (d) None of the above
_________ amounts to rejection of the original offer. (a) Cross offer (b) Special offer (c) Standing offer (d) Counter offer
A advertises to sell his old car by advertising in a newspaper. This offer is caleed: (a) General Offer (b) Special Offer (c) Continuing Offer (d) None of the above
In case a counter offer is made, the original offer stands: (a) Rejected (b) Accepted automatically (c) Accepted subject to certain modifications and variations (d) None of the above
In case of unenforceable contract having some technical defect, parties (a) Can sue upon it (b) Cannot sue upon it (c) Should consider it to be illegal (d) None of the above
If entire specified goods is perished before entering into contract of sale, the contract is (a) Valid (b) Void (c) Voidable (d) Cancelled
______________ contracts are also caled contracts with executed consideration. (a) Unilateral (b) Completed (c) Bilateral (d) Executory
A offers B to supply books @ Rs 100 each but B accepts the same with condition of 10% discount. This is a case of (a) Counter Offer (b) Cross Offer (c) Specific Offer (d) General Offer
_____________ is a game of chance. (a) Conditional Contract (b) Contingent Contract (c) Wagering Contract (d) Quasi Contract
There is no binding contract in case of _______ as one's offer cannot be constructed as acceptance (a) Cross Offer (b) Standing Offer (c) Counter Offer (d) Special Offer
An offer is made with an intention to have negotiation from other party. This type of offer is: (a) Invitation to offer (b) Valid offer (c) Voidable (d) None of the above
When an offer is made to the world at large, it is ____________ offer. (a) Counter (b) Special (c) General (d) None of the above
Implied contract even if not in writing or express words is perfectly _______________ if all the conditions are satisfied:- (a) Void (b) Voidable (c) Valid (d) Illegal
A specific offer can be accepted by ___________. (a) Any person (b) Any friend to offeror (c) The person to whom it is made (d) Any friend of offeree
An agreement toput a fire on a person's car is a ______: (a) Legal (b) Voidable (c) Valid (d) Illegal
"Holiday Packages" announced as an advertisement are an example of _________: (a) Offer (b) Counter Offer (c) Invitation to Offer (d) None of the above
A match fixing contract between a player and a broker is a: (a) Valid Contract (b) Unenforceable Contract (c) Void Contract (d) Illegal Contract
When a bookseller sells a book on cash payment then it is called as ___________: (a) Unilateral Contract (b) Bilateral Contract (c) Executed Contract (d) Executory Contract
____________ agreements are created by situation: (a) Written (b) Oral (c) Void (d) Implied
Cache = 0.015625 Seconds