Does FastMM support reserving virtual memory and calling in chunks to grow an array?

Question

I know I can reserve virtual memory using VirtualAlloc.
e.g. I can claim 1GB of virtual memory and then call in the first MB of that to put my a growing array into.
When the array grows beyond 1MB I call in the 2nd MB and so on.
This way I don't need to move the array around in memory when it grows, it just stays in place and the Intel/AMD virtual memory manager takes care of my problems.

However does FastMM support this structure, so I don't have to do my own memory management?

Pseudo code:

type
  PBigarray = ^TBigarray;
  TBigArray = array[0..0] of SomeRecord;

....

begin
  VirtualMem:= FastMM.ReserveVirtualMemory(1GB);
  PBigArray:= FastMM.ClaimPhysicalMemory(VirtualMem, 1MB);
....

procedure GrowBigArray
begin
  FastMM.ClaimMorePhysicalMemory(PBigArray, 1MB {extra});
  //will generate OOM exception when claim exceeds 1GB

Does FastMM support this?

Answer 1

No, FastMM4 (as of the latest version I looked at) does not explicitly support this. It's really not a functionality you would expect in a general purpose memory manager as it's trivially simple to do with VirtualAlloc calls.

NexusMM4 (which is part of NexusDB) does something that gives you a similar result, but without wasting all the address space before it is needed in the background.

If you make an initial large allocation (directly via GetMem, or indirectly via a dynamic array or such) the memory is allocated in just the size needed, via VirtualAlloc.

But if that allocation is then resized to a larger size, NexusMM will use a different way to allocate memory which allows it to simply unmap the allocation from the address space an remap it again, at a larger size, when further reallocs takes place.

This prevents the 2 major problems that most general purpose memory managers have when reallocating:

• during a normal realloc the existing and new allocation need to be present in the address space at the same time, temporarily doubling the address space and physical memory requirements
• during a normal realloc, the whole contents of the existing allocation needs to be copied

So with NexusMM you would get all the advantages of what you showed in your pseudo code (with the exception that the first realloc will involve a copy, and that growing your array might change it's address) by simply using normal GetMem/ReallocMem/FreeMem calls.