Shadow memory
Encyclopedia
Shadow memory describes a computer science
technique in which potentially every byte
used by a program during its execution has a shadow byte or bytes. These shadow bytes are typically invisible to the original program and are used to record information about the original piece of data. The program is typically kept unaware of the existence of shadow memory by using a dynamic binary translator
/instrumentor, which, among other things, may translate the original programs memory read and write operations into operations that do the original read and write and also update the shadow memory as necessary.
This technique when implemented naively has both high slowdown and relatively large RAM
requirements. The shadow memory requirements can be lessened through two level tables, similar to those used by modern operating system
s for virtual memory
lookup. For example, initially all first level page entries (each entry might cover 128 kB
) might point to a reserved "invalid" or "uninitialized" page entry. As the program reads/writes to/from a section specified by a page entry, then a new page entry is automatically created and initialized with the default values. Future reads/writes to that section will use that newly created entry.
The slowdown is harder to overcome, as each load and store must somehow result in the shadow memory being updated if 100% correctness is desired. Furthermore, modern CPUs
perform relatively poorly when doing data intensive tasks due to the limits of local cache
sizes.
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
technique in which potentially every byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
used by a program during its execution has a shadow byte or bytes. These shadow bytes are typically invisible to the original program and are used to record information about the original piece of data. The program is typically kept unaware of the existence of shadow memory by using a dynamic binary translator
Binary translation
In computing, binary translation is the emulation of one instruction set by another through translation of code. Sequences of instructions are translated from the source to the target instruction set...
/instrumentor, which, among other things, may translate the original programs memory read and write operations into operations that do the original read and write and also update the shadow memory as necessary.
This technique when implemented naively has both high slowdown and relatively large RAM
Ram
-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...
requirements. The shadow memory requirements can be lessened through two level tables, similar to those used by modern operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s for virtual memory
Virtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
lookup. For example, initially all first level page entries (each entry might cover 128 kB
Kilobyte
The kilobyte is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol KB have historically been used to refer to either 1024 bytes or 1000 bytes, dependent upon context, in the fields of computer science and information...
) might point to a reserved "invalid" or "uninitialized" page entry. As the program reads/writes to/from a section specified by a page entry, then a new page entry is automatically created and initialized with the default values. Future reads/writes to that section will use that newly created entry.
The slowdown is harder to overcome, as each load and store must somehow result in the shadow memory being updated if 100% correctness is desired. Furthermore, modern CPUs
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
perform relatively poorly when doing data intensive tasks due to the limits of local cache
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
sizes.