memory - Vulkan's VkAllocationCallbacks implemented with malloc/free()

Question

I'm reading Vulkan Memory Allocation - Memory Host and seems that VkAllocationCallbacks can be implemented using naive malloc/realloc/free functions.

typedef struct VkAllocationCallbacks {
   void*                                   pUserData;
   PFN_vkAllocationFunction                pfnAllocation;
   PFN_vkReallocationFunction              pfnReallocation;
   PFN_vkFreeFunction                      pfnFree;
   PFN_vkInternalAllocationNotification    pfnInternalAllocation;
   PFN_vkInternalFreeNotification          pfnInternalFree;
} VkAllocationCallbacks;

But I see only two possible reasons to implement my own vkAllocationCallback:

• Log and track memory usage by Vulkan API;
• Implement a kind of heap memory management, it is a large chunk of memory to be used and reused, over and over. Obviously, it can be a overkill and suffer same sort of problems of managed memory (as in Java JVM).

Am I missing something here ? What sort of applications would worth implementing vkAllocationCallbacks ?

Answer 1

From the spec: "Since most memory allocations are off the critical path, this is not meant as a performance feature. Rather, this can be useful for certain embedded systems, for debugging purposes (e.g. putting a guard page after all host allocations), or for memory allocation logging."

With an embedded system, you might have grabbed all the memory right at the start, so you don't want the driver calling malloc because there might be nothing left in the tank. Guard pages and memory logging (for debug builds only) could be useful for the cautious/curious.

I read on a slide somewhere (can't remember where, sorry) that you definitely should not implement allocation callbacks that just feed through to malloc/realloc/free because you can generally assume that the drivers are doing a much better job than that (e.g. consolidating small allocations into pools).

I think that if you're not sure whether you ought to be implementing allocation callbacks, then you don't need to implement allocation callbacks and you don't need to worry that maybe you should have.

I think they're there for those specific use cases and for those who really want to be in control of everything.