CODEWITHSUNDEEP
c++memorymemory-management
Mortano
Mortano

Reputation: 300

Win7 C++ application always reserving at least 4k memory per allocation

I'm currently looking into memory consumption issues of a C++ application that I have written (a rendering engine using OpenGL) and have stumbled upon a rather unusual problem: I'm using my own allocators basically everywhere in the system, which all obtain their memory from a default allocator which is using malloc()/free() for the actual memory.

It turns out that my application is always reserving at least 4096 bytes (the page size on my system) for every allocation through malloc(), even if the size is significantly smaller. malloc(8) or even malloc(1) both result in an increase of memory of 4096 bytes. I'm tracking the used memory size through GetProcessMemoryInfo() directly before and after the allocation, as well as through the TaskManager (which basically shows the same values). Interestingly, using _msize(ptr) returns the correct size of the pointer.

I can only reproduce this behaviour within my own application, testing it with a new VS2012 C++ project did not yield the same results. This behaviour also seems independent of the current reserved size of the application, even with more than 10GB of free RAM it always reserves at least 4K per allocation.

I have no deep knowledge of the innards of the Windows operating system (if it is at all related to the OS), so if anyone has an idea what could cause this behaviour I would be greatful!

Upvotes: 1

Views: 160

Answers (2)

mark
mark

Reputation: 5469

You might be running with "full page heap"... a diagnostic mode to help more quickly catch memory access errors in your code.

Upvotes: 0

Max Lambertini
Max Lambertini

Reputation: 3719

Check this, it's from 1993 :-)

http://msdn.microsoft.com/en-us/library/ms810603.aspx

This does not mean that the smallest amount of memory that can be allocated in a heap is 4096 bytes; rather, the heap manager commits pages of memory as needed to satisfy specific allocation requests. If, for example, an application allocates 100 bytes via a call to GlobalAlloc, the heap manager allocates a 100-byte chunk of memory within its committed region for this request. If there is not enough committed memory available at the time of the request, the heap manager simply commits another page to make the memory available.

Upvotes: 2

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