05/11/20

## Dynamic Partitioning Management

### Allocating Available Memory

#### First Fit

> * First fit starts scanning **from the start** of the linked list until a link is found, which can fit the process
>   * If the requested space is **the exact same size** as the 'hole', all the space is allocated
>   * Otherwise the free link is split into two:
>     * The first entry is set to the **size requested** and marked **used**
>     * The second entry is set to **remaining size** and **free**.

#### Next Fit

> * The next fit algorithm maintains a record of where it got to last time and restarts it's search from there
>   * This gives an even chance to all memory to get allocated (first fit concentrates on the start of the list)
> * However simulations have been run which show that this is worse than first fit.
>   * This is because a side effect of **first fit** is that it leaves larger partitions towards the end of memory, which is useful for larger processes.

#### Best Fit

> * The best fit algorithm always **searches the entire linked list** to find the smallest hole that's big enough to fit the memory requirements of the process.
>   * It is **slower** than first fit
>   * It also results in more wasted memory. As a exact sized hole is unlikely to be found, this leaves tiny (and useless) holes.
>
> Complexity: $O(n)$

#### Worst Fit

> Tiny holes are created when best fit split an empty partition.
>
> * The **worst fit algorithm** finds the **largest available empty partition** and splits it.
>   * The **left over partition** is hopefully **still useful**
>   * However simulations show that this method **isn't very good**.
>
> Complexity: $O(n)$ 

#### Quick Fit

> * Quick fit maintains a **list of commonly used sizes**
>   * For example a separate list for each of 4 Kb, 8 Kb, 12 Kb etc holes
>   * Odd sized holes can either go into the nearest size or into a special separate list.
> * This is much f**aster than the other solutions**, however similar to **best fit** it creates **many tiny holes**.
> * Finding neighbours for **coalescing** (combining empty partitions) becomes more difficult & time consuming.

### Coalescing

Coalescing (join together) takes place when **two adjacent entries** in the linked list become free.

* Both neighbours are examined when a **block is freed**
  * If either (or both) are also **free** then the two (or three) **entries are combined** into one larger block by adding up the sizes
    * The earlier block in the linked list gives the **start point**
    * The **separate links are deleted** and a **single link inserted**.

### Compacting

Even with coalescing happening automatically, **free blocks** may still be **distributed across memory**

> * Compacting can be used to join free and used memory
> * However compacting is more **difficult and time consuming** to implement then coalescing.
>   * Each **process is swapped** out & **free space coalesced**.
>   * Processes are swapped back in at lowest available location.

## Paging

Paging uses the principles of **fixed partitioning** and **code re-location** to devise a new **non-contiguous management scheme**

> * Memory is split into much **smaller blocks** and **one or multiple blocks** are allocated to a process (e.g. a 11 Kb process would take 3 blocks of 4 Kb)
> * These blocks **do not have to be contiguous in main memory**, but **the process still perceives them to be contiguous**
> * Benefits:
>   * **Internal fragmentation** is reduced to the **last block only** (e.g. previous example the third block, only 3 Kb will be used)
>   * There is **no external fragmentation**, since physical blocks are **stacked directly onto each other** in main memory.

![Paging in main memory](/lectures/osc/assets/L.png)

![Paging processor's view](/lectures/osc/assets/M.png)

![Paging](/lectures/osc/assets/P.png)

A **page** is a **small block** of **contiguous memory** in the **logical address space** (as seen by the process)

* A **frame** is a **small contiguous block** in **physical memory**.
* Pages and frames (usually) have the **same size**:
  * The size is usually a power of 2.
  * Size range between 512 bytes and 1 Gb. (most common 4 Kb pages & frames)

**Logical address** (page number, offset within page) needs to be **translated** into a **physical address** (frame number, offset within frame)

* Multiple **base registers** will be required
  * Each logical page needs a **separate base register** that specifies the start of the associated frame
  * i.e a **set of base registers** has to be maintained for each process
* The base registers are stored in the **page table**

![Mapping page tables](/lectures/osc/assets/Q.png)

The page table can be seen as a **function**, that **maps the page number** of the logical address **onto the frame number** of the physical address
$$
frameNumber = f(pageNumber)
$$

* The **page number** is used as an **index to the page table** that lists the **location of the associated frame**.
* It is the OS' duty to maintain a list of **free frames**.

![address translation](/lectures/osc/assets/R.png)

We can see that the **only difference** between the logical address and physical address is the **4 left most bits** (the **page number and frame number**). As **pages and frames are the same size**, then the **offset value will be the same for both**. 

This allows for **more optimisation** which is important as this translation will need to be **run for every memory read/write** call.
