Exec Memory Pools

A memory pool is a block of memory that an application allocates for all its memory needs. The application draws from this pool rather than the system's free memory list whenever it requires memory.

Memory pools add to the efficiency of the system and applications in two ways. The first is a potential decrease in memory fragmentation. Memory fragmentation is a condition where system memory is made up of blocks of allocated memory interspersed with blocks of free memory. The second benefit of memory pools is faster memory allocations and deallocations.

By having an application take a pool of memory and allocate from it, fragmentation of system memory is decreased. An application may require six allocations ranging from 20 to 678 bytes in size which can be scattered throughout system memory. However, if those same six allocations are taken from a pool, the fragmentation occurs within the pool, but as far as the system is concerned, it's missing one large block instead of six small ones.

An application using a memory pool will have faster memory allocations because the memory list of a pool is smaller, and therefore easier to traverse than the memory list of the system memory. For deallocations, it's even faster because deleting the pool itself deletes all the individual allocations made from it instead of having to deallocate each piece that was allocated.

Memory Pool Organization

A memory pool consists of units called puddles. Other than that, it has no formal organization. There is no pool structure defined anywhere. All you know about a pool is its address. Exec's pool manager handles the rest of the details.

When you create a pool, you specify the size of its puddles and a threshold value, not the size of the pool. This is because memory pools are dynamic, they have no fixed size. The pool manager takes the memory for your application from the puddles. As you require memory, the pool manager expands the pool by adding puddles, and as you free memory, the pool manager shrinks the pool by deleting puddles.

The size of the puddles is important because the pool manager will try to use as much of a puddle as possible before adding a new puddle. Each time you allocate memory from the pool, the pool manager takes the memory from a puddle unless an allocation exceeds the amount of memory remaining in a puddle. If the allocation request exceeds the memory remaining in a puddle, the pool manager adds a new puddle. If the allocation exceeds the pool's puddle size, the pool manager will create a special oversized puddle to accommodate the application.

The key is to use all the drops in a puddle. If you create a pool with puddles of 200 bytes and allocate 150 bytes at a time, you'll waste 50 bytes in every puddle. Set the puddle size in accordance with your memory requirements.

The threshold value is the size of the largest allocation to use within a single puddle. An allocation request larger than the threshold value causes the pool manager to add a new puddle. Ideally, the threshold value should be the size of the largest allocation you will make.