Writable +Data Paging Overview

Purpose

Writable +data paging allows the demand paging of any user-side data (for example thread +stacks and heaps). Writable data paging makes use of a fixed size backing +store to page out data. The backing store may use a NAND flash partition or +an embedded MMC (eMMC).

The aim of data paging is to enable more memory-hungry +use cases without increasing the amount of physical RAM present in the device.

For +example, data paging enables:

running applications +that are not designed with the memory constraints of embedded devices in mind, +for example applications ported from other environments with writable data +paging (such as a PC).
running multiple applications +at the same time where previously there would not have been enough memory.

Data paging offers no performance increase, unlike code paging +which can speed up application loading by reading only the necessary pages +into RAM.

However, writable data paging does have +the following limitations:

It is only possible +to page the following types of memory:
- user heaps,
- user thread stacks,
- private chunks,
- global chunks,
- static data.
A single memory object +(e.g. a heap, stack, DLL or chunk) is the smallest granularity at which paging +can be configured
No attempt will be made +to page kernel-side data
No attempt will be made +to implement memory mapped files (e.g. posix's mmap)
No attempt will be made +to implement any kind of paging on the emulator.

Required background

This +document assumes that the reader is familiar with the concept of Demand +Paging, and the existing implementation of ROM and code paging in Symbian +platform.

Key concepts +and terms

Page: Memory is managed in fixed size units called pages. The size of a page +is usually determined by the hardware, and for ARM architectures this is 4K.
Paged in or Paged out: If a given page of memory is present in RAM it is said to be paged +in (or just 'present'), as opposed to paged out.
Paging in: The process of moving a page of memory from being paged out to being +paged in.
Paging out: The process of moving a page of memory from being paged in to being +paged out.
backing store: The external media used to hold paged out pages is referred +to as the swap area. This area may be much larger than physical RAM, +although a factor of around twice as large is considered normal in existing +systems.
Working set: The term working set is defined as the memory accessed during +a given time period.
Pinning: Pinning pages refers to paging in demand-paged memory and forcing +it to remain in RAM until it is unpinned at a later time.

Classes

The +following classes are affected by the use of writable data demand paging:

The +following classes are involved in the use of threads and processes:

+ + + +

Class

Description

+ + +

RThread

This class is used to handle threads.

+ + +

RProcess

This class is used to handle processes.

+ + +

TThreadCreateInfo

Used to specify the attributes of a new thread.

+ + + +

These classes are used in handling memory:

+ + + +

Class

Description

+ + +

RChunk

Handle a chunk.

+ + +

TChunkCreateInfo

A structure that specifies the type and properties of the chunk +that is to be created.

+ + +

TChunkHeapCreateInfo

A structure that specifies the type and properties of a chunk with +a heap within it.

+ + +

UserHeap

A set of functions that are used to create heaps.

+ + + +

The following classes are used in client/server communication:

+ + + +

Class

Description

+ + +

CServer2

The abstract base class for servers.

+ + +

TIpcArgs

A client/server class that clients use to package arguments that +are to be sent to a sever.

+ + + +