13/11/20 ## Virtual Memory & Potential Problems #### Page Replacement ##### Second chance > - If a page at the front of the list has **not been referenced** it is **evicted** > - If the reference bit is set, the page is **placed at the end** of the list and its reference bit is unset. > - This works better than FIFO and is relatively simple > - **Costly to implement** as the list is constantly changing. > - Can degrade to FIFO if all pages were initially referenced. ##### Clock Replacement Algorithm > The second chance implementation can be improved by **maintaining the page list as a circle** > > - A **pointer** points to the last visited page. > - In this form, the algorithm is called the one-handed clock > - It is faster, but can still be **slow if the list is long**. > - The **time spent** on **maintaining** the list is **reduced**. ![clock replacement](assets/a2.png) ##### Not Recently Used (NRU) > For NRU, **referenced** and **modified** bits are kept in the page table > > - Referenced bits are set to 0 at the start, and **reset periodically** > > There are four different **page types** in NRU: > > 1. Not referenced recently, not modified > 2. Not referenced recently, modified > 3. Referenced recently, not modified > 4. Referenced recently, modified > > **Page table entries** are inspected upon every **page fault**. This could be implemented in the following way > > 1. Find a page from **class 0** to be removed. > 2. If step 1 fails, scan again looking for **class 1**. During this scan we set the reference bit to 0 on each page that is bypassed > 3. If step 2 fails, start again from step 1 (Now we should find pages from class 2&3 have been moved to class 0 or 1) > > The NRU algorithm provides a **reasonable performance** and is easy to understand and implement. ##### Least Used Recently > Least recently used **evicts the page** that has **not been used for the longest** > > - The OS must keep track of when a page was last used. > - Every page table entry contains a field for the counter > - This is **not cheap to implement** as we need to maintain a **list of pages** which are **sorted** in the order in which they have been used. > > This algorithm can be **implemented in hardware** using a **counter** that is incremented after each instruction ... ![least used recently visualisation](assets/a3.png) This will look familiar to the FIFO algorithm. However, when a page is used, it is treated as if it has just come in. ### Resident Set How many pages should be allocated to individual processes: - **Small resident sets** enable us to store **more processes in memory** => improved CPU utilisation. - **Small resident sets** may result in **more page faults** - **Large resident sets** may **no longer reduce** the **page fault rate** (**diminishing returns**) A trade-off exists between the **sizes of the resident sets** and **system utilisation**. Resident set sizes may be **fixed** or **variable** (adjusted at run-time) - For **variable-sized** resident sets, **replacement policies** can be: - **Local**: a page of the same process is replaced - **Global**: a page can be taken away from a **different process** - Variable sized sets require **careful evaluation of their size** when a **local scope** is used (often based on the **working set** or the **page fault rate**) ### Working Set The **resident set** comprises the set of pages of the process that are in memory (they have a corresponding frame) The **working set** is a subset of the resident set that is actually needed for execution. - The **working set** $W(t, k)$ comprises the set of referenced pages in the last $k$ (working set window) **virtual time units for the process**. - $k$ can be defined as **memory references** or as **actual process time** - The set of most recently used pages - The set of pages used within a pre-specified time interval - The **working set size** can be used as a guide for the number of frames that should be allocated to a process. ![working set](assets/a4.png) The working set is a **function of time** $t$: - Processes **move between localities**, hence, the pages that are included in the working set **change over time** - **Stable** intervals alternate with intervals of **rapid change** $|W(t,k)|$ is then a variable in time. Specifically: $$ 1\le |W(t,k)| \le min(k, N) $$ where $N$ is the total number of pages of the process. All the maths is saying is that the size of the working set can be as small as **one** or as large as **all the pages in the process**. Choosing the right value for $k$ is important: - Too **small**: inaccurate, pages are missing - Too **large**: too many unused pages present - **Infinity**: all pages of the process are in the working set Working sets can be used to guide the **size of the resident sets** - Monitor the working set - Remove pages from the resident set that are not in the working set The working set is costly to maintain => **page fault frequency (PFF)** can be used as an approximation: $PFF\space\alpha\space k$ - If the PFF is increased -> we need to increase $k$ - If PFF is very low -> we could decrease $k$ to allow more processes to have more pages. #### Global Replacement > Global replacement policies can select frames from the entire set (they can be taken from other processes) > > - Frames are **allocated dynamically** to processes > - Processes cannot control their own page fault frequency. The PFF of one process is **influenced by other processes**. #### Local Replacement > Local replacement policies can only select frames that are allocated to the current process > > - Every process has a **fixed fraction of memory** > - The **locally oldest page** is not necessarily the **globally oldest page** Windows uses a variable approach with local replacement. Page replacement algorithms can use both policies. ### Paging Daemon It is more efficient to **proactively** keep a number of **free pages** for **future page faults** - If not, we may have to **find a page** to evict and **write it to the drive** (if it's been modified) first when a page fault occurs. Many systems have a background process called a **paging daemon**. - This process **runs at periodic intervals** - It inspects the state of the frames and if too few frames are free, it **selects pages to evict** (using page replacement algorithms) Paging daemons can be combined with **buffering** (free and modified lists) => write the modified pages **but keep them in main memory** when possible. **Buffering**: a process that preemptively writes modified pages to the disk. That way when there's a page fault we don't lose the time taken to write to disk ### Thrashing Assume **all available pages are in active use** and a new page needs to be loaded: - The page that will be evicted will have to be **reloaded soon afterwards** **Thrashing** occurs when pages are **swapped out** and then **loaded back in immediately** #### Causes of thrashing include: - The degree of multi-programming is too high, i.e. the total **demand** (the sum of all working set sizes) **exceeds supply** (the available frames) - An individual process is allocated **too few pages** This can be prevented by **using good page replacement algorithms**, reducing the **degree of multi-programming** or adding more memory. The **page fault frequency** can be used to detect that a system is thrashing. > - CPU utilisation is too low => scheduler **increases degree of multi-programming** > - Frames are allocated to new processes and taken away from existing processes > - I/O requests are queued up as a consequence of page faults > > This is a positive reinforcement cycle. When all this comes together, this is how memory management works in modern computers.