Simpool

Simpool is a very simple pooled memory allocator that offers recipes for use in C++ by overloading ::operator new(std::size_t) and fulfilling an STL allocator concept.

Background

The concept behind a pooled memory allocator is to reduce the number of system calls to allocate memory, and instead takes memory from an already allocated segment of memory. It can be more efficient if there are many small allocations or if the allocator function incurs significant overhead.

Why do we need another memory pool?

While there are many other existing pool implementations, all the implementations that I can find have a flaw. This code addresses the following weaknesses:

1. While others create a usable pool for the memory, they do not for keeping track of the blocks, meaning these allocations still incur the overhead of the system malloc for each block. This seems like an oversight.

2. Other implementations do not offer the ability to select the memory spaces used for both the memory pool internally and for the memory pointers provided by the class.

Design

This code uses a series of pools to represent the internal and allocated memory. These pools can be in any memory space reachable from the thread allocation and deallocation function.

Fixed Type

The FixedSizePool<T, MA, NP> class stores "pools" each of NP*sizeof(unsigned int)*8 objects of type T in the memory space with allocator struct MA. An example of an allocator struct for the system malloc()/free() methods:

struct CPUAllocator
{
  static inline void *allocate(std::size_t size) { return std::malloc(size); }
  static inline void deallocate(void *ptr) { std::free(ptr); }
};

The class keeps track of which locations in the pool are unused by flipping single bits. It can do this because the objects are all the same size.

This algorithm calls the allocator function once per pool creation, so is guaranteed to call it no more than every NP*sizeof(unsigned int)*8 allocations.

The public non-constructor/destructor methods are:

• T* allocate(): returns a pointer to memory for an object T
• void deallocate(T* ptr): Tells the pool that ptr will no longer be used. The behavior is undefined if ptr was not returned from allocate() above.
• std::size_t allocatedSize() const: Return the allocated size, without internal overhead.
• std::size_t totalSize() const: Return the total size of all allocations within.
• std::size_t numPools() const: Return the number of fixed size pools.

Dynamic Type

The DynamicSizePool<MA, IA, MINSIZE> class allocates objects with the MA allocator struct, and internally keeps track of the blocks using a FixedMemoryPool using the IA allocator struct. Each of the blocks allocated with MA are at least size MINSIZE - smaller allocations are carved out.

This class largely follows the algorithm used in cnmem. This involves splitting blocks if allocations are smaller than MINSIZE, and merging blocks if possible when memory is marked as no longer used. It is therefore difficult to determine an upper bound on the number of allocations made with IA and MA.

The public non-constructor/destructor methods are:

• void* allocate(std::size_t size): returns a pointer to size bytes of memory.
• void deallocate(T* ptr): Tells the pool that ptr will no longer be used. The behavior is undefined if ptr was not returned from allocate(std::size_t) above.
• std::size_t allocatedSize() const: Return the allocated size, without internal overhead.
• std::size_t totalSize() const: Return the total size of the class and all allocations within.
• std::size_t numFreeBlocks() const: Return the number of free blocks.
• std::size_t numUsedBlocks() const: Return the number of used blocks.

License

© Copyright 2017 IBM Corporation. MIT License.

Authors

• Johann Dahm <johann.dahm@ibm.com> Primary contact

See also

• GNU glibc obstack: info page
• NVidia cnmem: Github page
• Google Perftools tcmalloc: website
• jemalloc: website