Top 10 List of Week 05

1. What is Virtual Memory?
   Linux supports virtual memory, that is, using a disk as an extension of RAM so that the effective size of usable memory grows correspondingly. The kernel will write the contents of a currently unused block of memory to the hard disk so that the memory can be used for another purpose. When the original contents are needed again, they are read back into memory.

2. Creating a Swap Space
   A swap file is an ordinary file; it is in no way special to the kernel. The swap file reserves the disk space so that the kernel can quickly swap out a page without having to go through all the things that are necessary when allocating a disk sector to a file. This site has a tutorial for creating a swap space, check it out if you want to know.

3. Difference between Uniform Memory Access (UMA) and Non-uniform Memory Access (NUMA)
   This website explains the difference between UMA and NUMA. The explanation is very direct and simple, making it easier for us to understand the difference.

4. Demand Paging
   This site explains what is Page Fault and Thrashing. Page Fault occurs if the referred page is not present in the main memory then there will be a miss and the concept is called Page miss or page fault. Thrashing occurs if he number of page faults is equal to the number of referred pages or the number of page faults are so high so that the CPU remains busy in just reading the pages from the secondary memory then the effective access time will be the time taken by the CPU to read one word from the secondary memory and it will be so high. The concept is called thrashing.

5. The Right Way to Monitor Virtual Memory on Linux
   This site explains how to monitor virtual memory on linux. There are three things to measure a memory system, that is how much is physical memory is in use, how much virtual memory is in use, and the paging rate. We can use top or free to check memory usage. This site helps me understand on how to monitor memory usage on linux.

6. Page Table
   Page Table is a data structure used by the virtual memory system to store the mapping between logical addresses and physical addresses. Logical addresses are generated by the CPU for the pages of the processes therefore they are generally used by the processes. Physical addresses are the actual frame address of the memory. They are generally used by the hardware or more specifically by RAM subsystems.

7. Mapping from page table to main memory
   In operating systems, there is always a requirement of mapping from logical address to the physical address. However, this process involves various steps which are defined as follows. First, Generation of logical address. Second, Scaling. Third, Generation of physical Address. Lastly, Getting Actual Frame Number.

8. Page Table Entry
   Along with page frame number, the page table also contains some of the bits representing the extra information regarding the page. This site explains each bit about the page, and it has a illustration to make it easier for us to understand.

9. Belady’s Anomaly
   In the case of LRU and optimal page replacement algorithms, it is seen that the number of page faults will be reduced if we increase the number of frames. However, Balady found that, In FIFO page replacement algorithm, the number of page faults will get increased with the increment in number of frames. This is the strange behavior shown by FIFO algorithm in some of the cases. This is an Anomaly called as Belady’s Anomaly.

10. Paging vs Segmentation
    This site explains the difference between paging and segmentation. It uses table to make it easier for us to read the differences. One of the differences of paging and segmentation is OS is responsible for Paging and Compiler is responsible for Segmentation.