Question

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3. What are the advantage and drawbacks of Virtual Memory with paging?

4. Which kind of fragmentation (external or internal) will be influenced by the page size? What will happen if the page size goes smaller? What is the drawback(s) of small page size?

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Answer 1

3.

Virtual Memory is a storage mechanism which offers user an illusion of having a very big main memory. It is done by treating a part of secondary memory as the main memory. In Virtual memory, the user can store processes with a bigger size than the available main memory.

Therefore, instead of loading one long process in the main memory, the OS loads the various parts of more than one process in the main memory. Virtual memory is mostly implemented with demand paging and demand segmentation.

Advantages of Virtual Memory:
Here, are pros/benefits of using Virtual Memory:

Virtual memory helps to gain speed when only a particular segment of the program is required for the execution of the program.
It is very helpful in implementing a multiprogramming environment.
It allows you to run more applications at once.
It helps you to fit many large programs into smaller programs.
Common data or code may be shared between memory.
Process may become even larger than all of the physical memory.
Data / code should be read from disk whenever required.
The code can be placed anywhere in physical memory without requiring relocation.
More processes should be maintained in the main memory, which increases the effective use of CPU.
Each page is stored on a disk until it is required after that, it will be removed.
It allows more applications to be run at the same time.
There is no specific limit on the degree of multiprogramming.
Large programs should be written, as virtual address space available is more compared to physical memory.

Disadvantages of Virtual Memory:
Here, are drawbacks/cons of using virtual memory:

Applications may run slower if the system is using virtual memory.
Likely takes more time to switch between applications.
Offers lesser hard drive space for your use.
It reduces system stability.
It allows larger applications to run in systems that don't offer enough physical RAM alone to run them.
It doesn't offer the same performance as RAM.
It negatively affects the overall performance of a system.
Occupy the storage space, which may be used otherwise for long term data storage.

4.

In operating system internals and design principles (7th edition), there is a sentence "the smaller the page size the lesser the amount of internal fragmentation".

Let say your process has 12 bytes of executable code, 34 bytes of read only data and 56 bytes of read/write data. Note that each of these areas have different access permissions, and therefore (for most CPUs where permissions are associated with pages) they can't share pages.

If the page size is 100 bytes then the 12 bytes of executable code is going to have to consume 100 bytes (a whole page) and 88 bytes of that will be wasted (internal fragmentation); and the whole process will waste 198 bytes due to internal fragmentation (because the size of each of the 3 areas will need to be rounding up to the page size). If the page size is 16384 bytes then the same process would waste 49050 bytes; and if the page size was 10 bytes it'd waste 18 bytes.

More specifically; if N processes have M unique areas (where a unique area may be shared by multiple processes) you'd expect/assume that the average amount of bytes wasted due to internal fragmentation will be "M * PAGE_SIZE/2".

Rarely do processes require the use of an exact number of pages. As a result, the last page will likely only be partially full, wasting some amount of memory. Larger page sizes increase the potential for wasted memory this way, as more potentially unused portions of memory are loaded into main memory. Smaller page sizes ensure a closer match to the actual amount of memory required in an allocation.

large pages:
pro - smaller page table, less page faults, less overhead in reading/writing of pages
con - more internal fragmentation, worse locality of reference
smaller pages:
pro - reduces internal fragmentation, better with locality of reference

con - bigger page table, more page faults, overhead in reading/writing of pages