Operating Systems 2 - Memory Manager

Summary

Sean's tutorial explores various memory management schemes, starting with the inefficient single user contiguous allocation, moving to fixed and dynamic partitions, and then to the more efficient paged memory allocation. He discusses demand paging, segmented memory allocation, and their combination, emphasizing virtual memory's ability to extend beyond physical RAM limits. The tutorial highlights the evolution of memory management, aiming to improve CPU efficiency and job processing times.

Takeaways

• 😀 The tutorial introduces three old memory management schemes: single user contiguous, fixed partitions, and dynamic partitions.
• 🔄 Single user contiguous memory management was inefficient as it required the entire memory space for a single job and was impractical for modern systems.
• 📏 Fixed partitions allowed multiple jobs to be stored in memory but had static sizes, requiring a system reboot to change, leading to inefficiency.
• 🔄 Dynamic partitions improved on fixed partitions by allocating memory based on job requests, but still required contiguous blocks and efficient allocation strategies.
• 📚 The script discusses allocation strategies like first fit and best fit, which have different trade-offs in terms of memory waste and processing speed.
• 🗂️ Virtual memory is introduced as a solution to overcome the limitations of contiguous memory storage, allowing for more efficient memory usage.
• 📑 Paged memory allocation divides jobs into equal-sized pages, which can be stored in any available page frame, simplifying memory management.
• 🗺️ Demand paging is a variation of paged allocation where pages are loaded into memory only when needed, improving efficiency further.
• 🔄 Page replacement policies like FIFO, LRU, LFU, and MRU are crucial for managing which pages to keep in memory and which to swap out.
• 📈 Segmented memory allocation breaks jobs into variable-sized segments based on the program's structure, allowing for more logical memory usage.
• 🔄 Combined segmentation and paging allocation offers the benefits of both approaches, with the addition of segment map and page map tables for better control.
• 💾 Virtual memory allows systems to exceed physical RAM capacity by using the hard drive as backup memory, albeit at a slower speed but with increased flexibility.

Q & A

What was the first memory management scheme discussed in the tutorial?

The first memory management scheme discussed was single user contiguous memory management, which was not very efficient as it required a single job to reserve the entire memory space.

Why was the single user contiguous memory management scheme considered impractical for modern operating systems?

It was considered impractical because it would be very slow to process all of the jobs on demand, and if the job size was larger than the available RAM, it could not be executed.

What is the main advantage of fixed partitions over single user contiguous memory management?

Fixed partitions allow allocated partitions to contain more than a single job at one time, improving efficiency over single user contiguous memory management.

What is the main disadvantage of fixed partitions in memory management?

The main disadvantage is that the sizes of these partitions are static, meaning they cannot be changed unless the system is rebooted, which is not ideal for dynamic job processing.

How does dynamic partitioning improve upon fixed partitioning?

Dynamic partitioning improves by allocating only as much memory as jobs request when they are loading for processing, thus avoiding memory waste and not requiring a system reboot to change partition sizes.

What are the two methods for allocating jobs to available partitions in fixed and dynamic partitions?

The two methods are first fit, which finds the first partition with sufficient storage space, and best fit, which looks for the smallest partition to minimize memory waste.

What is the purpose of the paged memory allocation scheme?

The purpose of paged memory allocation is to divide the job into equal-sized parts, allowing these parts to be stored in any available page frame in the main memory, improving efficiency over contiguous block storage.

What are the three tables used to keep track of page locations in paged memory allocation?

The three tables are the job table, the page map table, and the memory map table, which keep track of various aspects of page locations and statuses.

How does demand paging differ from paged memory allocation?

Demand paging is similar to paged memory allocation, but the main difference is that pages are loaded on demand, meaning they are only brought into memory when they are actually needed.

What is the role of a page replacement policy in demand paging?

A page replacement policy determines which pages to replace when the memory is full, using algorithms such as first-in first-out, least recently used, least frequently used, and most recently used.

What is the main idea behind segmented memory allocation?

Segmented memory allocation splits the job into separate segments that reflect the nature and structure of the code, allowing for more logical and efficient memory usage compared to paged allocation.

How does the final memory allocation scheme combine the benefits of segmentation and paging?

The final scheme is a mixture of segmented and demand paging memory allocation, using both the segment map table and page map table, along with additional authority information, to offer the logical benefits of segmentation and the physical benefits of paging.

Why is virtual memory considered beneficial despite the increased processing cost?

Virtual memory is beneficial because it allows the system to exceed the physical memory capacity by using the hard drive as a backup memory, acting as an extension to the RAM and providing a more efficient service to the CPU for handling jobs.