C++ and linux kernel memory allocation

kernel memory allocation

buddy algorithm
slab allocator

The slab allocator and buddy algorithm are two different memory allocation techniques used in the Linux kernel, each with its own strengths and use cases.

Buddy Algorithm

The buddy algorithm is used for allocating memory in large contiguous blocks. It works by dividing memory into blocks of size 2^n and maintaining a binary tree structure. When a memory request is made, the allocator finds the smallest block that can satisfy the request. If a larger block is found, it is split into smaller blocks (buddies) until the desired size is obtained. This method is efficient for allocating large blocks of memory and helps reduce fragmentation](https://www.tutorialspoint.com/allocating-kernel-memory-buddy-system-and-slab-system).

Slab Allocator

The slab allocator, on the other hand, is designed for allocating small, frequently used objects. It maintains caches of pre-allocated memory chunks (slabs) for different object sizes. When an object is needed, it is allocated from the appropriate slab, which reduces the overhead of frequent allocations and deallocations. This method is particularly useful for kernel objects like process descriptors, file descriptors, and other small data structures](https://www.tutorialspoint.com/allocating-kernel-memory-buddy-system-and-slab-system).

Relationship

The relationship between the two lies in their complementary roles within the Linux kernel. The buddy algorithm is used for allocating large contiguous blocks of memory, which can then be subdivided by the slab allocator for smaller, frequently used objects. This combination allows the Linux kernel to efficiently manage memory for both large and small allocations, optimizing performance and reducing fragmentation](https://www.tutorialspoint.com/allocating-kernel-memory-buddy-system-and-slab-system).

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which algorithm does linux use when brk() syscall is called to allocate memory ?

When the brk() system call is used in Linux to allocate memory, it primarily relies on the buddy algorithm for managing the memory. The brk() system call adjusts the end of the data segment of a process, effectively increasing or decreasing the amount of memory allocated to the process. The buddy algorithm is used to allocate large contiguous blocks of memory, which can then be managed by the process.

Difference between malloc and new in c++ ?

malloc is a system call function. When calling malloc it will not call constructor of class or do memory initialization. free to free memory.

new is a primitive word in c++ itself. new will call constructor of class and do memory initialization. new is managed by c++ compiler. delete to free memory and destructor is called for delete

Memory allocation for process in linux

For small memory allocation brk is called.

brk will allocate memory from heap and the allocated memory will not be returned to kernel immediately after free. It will stay in the process memory mapping space for future use.
For large memory allocation mmap is called. For memory size allocation request > 128KB, memory allocated is in file mapping space in process memory space. The physical memory is returned to kernel immediately after free call which is different from brk

ref article about memory allocation