21.2.2. Page Buffering

In this lecture we will continue our discussion with paging. We looked at schemes to improve the performance of paging; in this we looked at page replacement algorithms, where better page replacement algorithms improve the performance of paging. Then we looked at the scheme of page buffering; which is related to the issue that when during replacement when we have to write to a dirty page, that dirty page has to be first written to disk and the system has to wait for this dirty page to be written out to disk and then only the page that is required can be brought into the memory frame.

In the context of computer memory management, page buffering is an important technique that helps avoid delays caused by reading from or writing to disk during page replacement. A dirty page is one that has been modified in memory but not yet saved back to the disk. If the operating system needs to replace a page in memory (due to memory constraints or page faults), it can lead to inefficiencies if it has to wait for this dirty page to be written to disk before it can load a new page. Buffering helps mitigate this waiting time.

Think of page buffering like a busy restaurant kitchen. When a chef needs to prepare a new dish (load a new page), but all the pots (memory frames) are being used, they first need to wash a pot (write a dirty page to disk). However, if they had a clean pot (free page) ready, they could simply start cooking (loading the new page) without any delays.

Now to avoid this waiting time, we keep a pool of free pages at any point in time so when we need to replace we as before we select a victim page, if that page is dirty we will write it to the disk, but, what we do is that instead of writing to this dirty page to disk first, we select a page from the free pool and allocate this for replacement.

To reduce the wait times associated with page replacement, operating systems maintain a pool of free pages. When a page needs to be replaced, if the page selected (the victim page) is dirty, it will indeed be written to disk. But instead of waiting for this write operation to complete, the system can immediately allocate a clean page from the pool to be used for the replacement. This allows for more efficient memory management as it effectively hides the time needed to write the dirty page to disk.

Imagine a takeaway restaurant where there is a stock of clean plates (the free pool). Instead of waiting to wash a dirty plate (write the dirty page), the staff can simply grab a clean plate from the stock whenever an order comes in (a page needs to be replaced). This keeps service swift and efficient.

After I have given this and the data has been written from the secondary storage into this frame in the free frame pool and the process has been restarted subsequent to service of the page-fault; then the I/O channel is free again and then what we do this victim page is written to the disk and then after this victim page is written to the disk, this page modified bit or the dirty bit for this page is again reset and then this frame is again added to the free frame pool.

Once the data from secondary storage is successfully loaded into the free frame and the process has resumed, the system can then proceed to write the victim page that was dirty back to disk. After this write operation is completed, the modified bit (or dirty bit), which indicates whether the page has been modified or not, is reset. This means that the page is now clean and can be added back to the free frame pool for future use.

Consider this like a library. Once a book (a victim page) is returned after being read (modified and written back), the librarian marks it as available for future loans (the dirty bit is reset). The book can now be borrowed again without any concerns.

A basic scheme was an extension to this basic scheme is that, what we do is that whenever I/O channel is free I find out a dirty page and I write it down into the disk so I all I maintain a queue of all the dirty pages that are currently there in memory and whenever the I/O channel is free, I slowly in my in the when the I/O channel is free I write it down into the disk and add these pages into the free frame pool.

An advanced technique within page buffering is maintaining a queue of dirty pages. When the I/O channel, which can handle data transfers to and from disk, is free, the operating system systematically goes through this queue and writes the dirty pages to disk. This takes advantage of any idle time to ensure that dirty pages are regularly cleared out, updating the disk storage without causing any delays for active processes.

Think of this like a team managing call-outs in a call center. Whenever an operator finishes their current call (the I/O channel is free), they immediately check to see if there are any messages in the queue that need to be replied to (dirty pages). They handle those queued messages sequentially to ensure no one is left waiting unnecessarily.

Now, I have added this to the free frame pool, I have added this frame into the free frame pool, but when I have written I have not destroyed the contents of this frame; if it so happens that my processor needs a page, which is there in the free frame pool and the then instead of going to the storage I can directly take that page from the free frame pool itself; again this issue what we are trying to say is that I have a page for which its dirty bit is on therefore, according to the earlier scheme in when the I/O channel is free, I/O processor is free I have written this page into the into the free frame pool sorry into the disk.

After writing a dirty page to disk and resetting its dirty bit, the contents of the page remain intact in the free frame pool. If the processor needs to access that particular page later, it can do so directly from the free frame pool, bypassing the need to go back to slower secondary storage. This quick access helps prevent unnecessary page faults, optimizing overall system performance.

Imagine a baker who has pre-prepared ingredients (free frame pool). When a cake needs to be decorated (page accessed), instead of going back to the pantry (secondary storage) for flour or sugar, they can grab pre-prepared ingredients right from their workbench (the free frame pool), saving time and effort.

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