Malloc vs custom allocator: Malloc has a lot of overhead. Why?
I have an image compression application that now has two different versions of memory allocation systems. In the original one, malloc is used everywhere, and in the second one, I implemented a simple pool-allocator, that just allocates chunk of memory and returns parts of that memory to myalloc() calls.
We've been noticing a huge memory overhead when malloc is used: At the height of its memory usage, the malloc() code requires about 170 megabytes of memory for a 1920x1080x16bpp image, while the pool allocator allocates just 48 megabytes, of which 47 are used by the program.
In terms of memory allocation patterns, the program allocates a lot of 8byte(most), 32-byte(many) and 1080byte-blocks(some) with the test image. Apart from these, there are no dynamic memory allocations in the code.
The OS of the testing system is Windows 7 (64 Bit).
How did we test memory usage?
With the custom allocator, we could see how much memory is used because all malloc calls are defered to the allocator. With malloc(), in Debug mode we just stepped through the code and watched the memory usage in the task manager. In release mode we did the same, but less fine grained because the compiler optimizes a lot of stuff away so we couldn't step through the code piece by piece (the memory difference between release and debug was about 20MB, which I would attribute to optimization and lack of debug information in release mode).
Could malloc alone be the cause of such a huge overhead? If so, what exactly causes this overhead inside malloc?
First at all malloc aligns the pointers to 16 byte boundaries. Furthermore they store at least one pointer (or allocated length) in the addresses preceding the returned value. Then they probably add a magic value or release counter to indicate that the linked list is not broken or that the memory block has not been released twice (free ASSERTS for double frees).
#include <stdlib.h>
#include <stdio.h>
int main(int ac, char**av)
{
int *foo = malloc(4);
int *bar = malloc(4);
printf("%dn", (int)bar - (int)foo);
}
Return: 32
On Windows 7 you will always get the low-fragmentation heap allocator, without explicitly calling HeapSetInformation() to ask for it. That allocator sacrifices virtual memory space to reduce fragmentation. Your program is not actually using 170 megabytes, you are just seeing a bunch of free blocks lying around, waiting for an allocation of a similar size.
This algorithm is very easy to beat with a custom allocator that doesn't do anything to reduce fragmentation. Which may well work out for you, albeit that you don't see the side effects of it until you keep the program running longer than a single debug session. You do need to make sure it is stable for days or weeks if that is the expected usage pattern.
Best thing to do is just not fret about it, 170 MB is rather small potatoes. And do keep in mind that this is virtual memory, it doesn't cost anything.
Caution: When you run your program in the Visual Studio or with any debugger attached, by default the malloc behaviour is changed a lot, Low Fragmentation Heap is not used and a memory overhead may be not representative of real usage (see also https://stackoverflow.com/a/3768820/16673). You need to use environment variable _NO_DEBUG_HEAP=1 to avoid being hit by this, or to measure the memory usage when not running under a debugger.
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