Demand +Paging Overview

Purpose

Demand +paging trades off increased available RAM against increased data latency, +increased media wear and increased power usage. Overall performance may be +increased as there can be more RAM available to each application as it runs.

Demand +paging is used to reduce the amount of RAM that needs to be shipped with a +device and so reduces the device cost.

Description

Demand +paging relies on the fact that most memory access is likely to occur in a +small region of memory and not spread over the entire memory map. This means +that if only that small area of memory is available to a process (or thread) +at any one time, then the amount of RAM required can be reduced. The small +area of memory is known as a page. The working RAM is broken down into pages, +some of which are used directly by the processor and the rest are spare. Since +Symbian platform is a multi-tasking operating system, there is more than one +page in the working RAM at any one time. There is one page per thread.

There +are three types of demand paging: ROM paging, code paging and writable data +paging. All of these types of demand paging have some features in common:

A free RAM pool
Working RAM
Paging Fault Handler
A source of code and/or +data that the processor needs to access.

With demand paging, the processor requires content to be present +in the working RAM. If the content is not present, then a 'paging fault' occurs +and the required information is loaded into a page of the working RAM. This +is known as "paged-in". When the contents of this page are no longer required, +then the page returns to the pool of free RAM. This is known as "paging-out".

The +difference between the types of demand paging is the source that is to be +used:

For ROM paging, it is +code and/or data stored under the ROM +file system
For code paging, it +is code and/or data stored using ROFS +file system
For writable data paging, +it is the writable data stored in RAM, for example user stacks and heaps. +In this case, the data is moved to a backing store.

+ +

The diagram above shows the basic operations involved in demand +paging:

The processor wants +to access content which is not available in the working RAM, causing a 'paging +fault'.
The paging fault handler +starts the process for copying a page of the source into a page of RAM.
A page is selected.
The contents of the +source is loaded into the page and this becomes part of the working memory.
When the contents of +the page are no longer required, the page is returned to the free RAM pool.

Demand Paging features

Demand Paging provides the +following features:

Reduced amount of RAM +required
Improved performance +in applications that involve loading a large amount of code into RAM, since +the number of memory access operations required has been reduced
Improved stability in +Out Of Memory (OOM) conditions.

Demand Paging limitations

The following are known +limitations of Demand Paging:

The access time cannot +be guaranteed.

There is an orders of magnitude difference in the access +time between an access where no page fault occurs and one where a page fault +occurs. If a page fault does not occur, then the time taken for a memory access +is in the tens to hundreds of nanosecond range. If a page fault does occur, +then the time taken for a memory access could be in the millisecond range
Device drivers have +to be written to allow for data latency when a page fault occurs.