Memory pool
Encyclopedia
Memory pools, also called fixed-size-blocks allocation, allow dynamic memory allocation comparable to malloc
or C++
's operator new. As those implementations suffer from fragmentation
because of variable block sizes, it can be impossible to use them in a real time system
due to performance. A more efficient solution is preallocating a number of memory blocks with the same size called the memory pool. The application can allocate, access, and free blocks represented by handles
at run time.
Many real-time operating systems use memory pools, such as the Transaction Processing Facility
.
Some systems, like the web server Nginx
, use the term memory pool to refer to a group of variable-size allocations which can be later deallocated all at once. This is also known as a region; see region-based memory management
.
with block sizes optimized for the application which deploys the module. The application can allocate, access and free memory with the following interface:
Drawbacks
Malloc
C dynamic memory allocation refers to performing dynamic memory allocation in the C via a group of functions in the C standard library, namely malloc, realloc, calloc and free....
or C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
's operator new. As those implementations suffer from fragmentation
Fragmentation (computer)
In computer storage, fragmentation is a phenomenon in which storage space is used inefficiently, reducing storage capacity and in most cases reducing the performance. The term is also used to denote the wasted space itself....
because of variable block sizes, it can be impossible to use them in a real time system
Real-time computing
In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...
due to performance. A more efficient solution is preallocating a number of memory blocks with the same size called the memory pool. The application can allocate, access, and free blocks represented by handles
Handle (computing)
In computer programming, a handle is a particular kind of smart pointer. Handles are used when application software references blocks of memory or objects managed by another system, such as a database or an operating system....
at run time.
Many real-time operating systems use memory pools, such as the Transaction Processing Facility
Transaction Processing Facility
TPF is an IBM real-time operating system for mainframes descended from the IBM System/360 family, including zSeries and System z9. The name is an initialism for Transaction Processing Facility....
.
Some systems, like the web server Nginx
Nginx
nginx is a Web server and a reverse proxy server for HTTP, SMTP, POP3 and IMAP protocols, with a strong focus on high concurrency, performance and low memory usage. It is licensed under a BSD-like license and it runs on Unix, Linux, BSD variants, Mac OS X, Solaris, and Microsoft Windows.- Overview...
, use the term memory pool to refer to a group of variable-size allocations which can be later deallocated all at once. This is also known as a region; see region-based memory management
Region-based memory management
In computer science, region-based memory management is a type of memory management in which each allocated object is assigned to a region. A region, also called a zone, arena, or memory context, is a collection of allocated objects that can be efficiently deallocated all at once...
.
Sample memory pool implementation
A simple memory pool module can allocate for example 3 pools at compile timeCompile time
In computer science, compile time refers to either the operations performed by a compiler , programming language requirements that must be met by source code for it to be successfully compiled , or properties of the program that can be reasoned about at compile time.The operations performed at...
with block sizes optimized for the application which deploys the module. The application can allocate, access and free memory with the following interface:
- To allocate memory from the pools. The function will determine the pool where the required block fits in. If all blocks of that pool are already reserved, the function tries to find one in the next bigger pool(s). An allocated memory block is represented with a handleHandle (computing)In computer programming, a handle is a particular kind of smart pointer. Handles are used when application software references blocks of memory or objects managed by another system, such as a database or an operating system....
. - To get an access pointer to the allocated memory.
- To free the formerly allocated memory block.
- The handle can for example be implemented with an unsigned int. The module can interpret the handle internally by dividing it into pool index, memory block index and a version. The pool and memory block index allow fast access to the corresponding block with the handle, while the version, which is incremented at each new allocation, allows detection of handles whose memory block is already freed (caused by handles retained too long).
Memory pool vs malloc
Benefits- Memory pools allow memory allocation with constant execution time (no fragmentationFragmentation (computer)In computer storage, fragmentation is a phenomenon in which storage space is used inefficiently, reducing storage capacity and in most cases reducing the performance. The term is also used to denote the wasted space itself....
). The memory release for thousands of objects in a pool is just one operation, not one by one if malloc is used to allocate memory for each object. - Memory pools can be grouped in hierarchical tree structures, which is suitable for special programming structures like loops and recursionRecursion (computer science)Recursion in computer science is a method where the solution to a problem depends on solutions to smaller instances of the same problem. The approach can be applied to many types of problems, and is one of the central ideas of computer science....
s. - Fixed-size block memory pools do not need to store allocation metadata for each allocation, describing characteristics like the size of the allocated block. Particularly for small allocations, this provides a substantial space savings.
Drawbacks
- Memory pools may need to be tuned for the application which deploys them.