Advantages vs disadvantages of vm.overcommit_memory=1 vs vm.overcommit_memory=0

Question

On Linux, if vm.overcommit_memory=1, you can allocate huge memory blocks, but if you use less, these will not affect memory usage.

Lets suppose following code:

const size_t size = 128;
void p = malloc(size);
process(p, size); // use up to size bytes

vs

const size_t HUGE_SIZE = 1ull * 1024ull * 1024ull * 1024ull; // 1 GB
const size_t size = 128;
void p = malloc(HUGE_SIZE);
process(p, size); // use up to size bytes

memory usage in both case will be "similar" (OK, may be 4 KB in second case, vs 128 bytes in first case)

• is second approach really takes 4 KB?
• is second approach slower?
• what if I have several 1000's blocks of 1 GB?
  what if I often allocate / deallocate these several 1000's blocks?
• any more disadvantages I can not see?
• I read MacOS support the same, any difference there?

Answer 1

is second approach really takes 4 KB?

In both cases it takes physical memory as much as accessed by process(), with one page granularity. Difference is process address space allocation.

is second approach slower?

It may be slower on searching for appropriate region in process address space.

what if I have several 1000's blocks of 1 GB?

On 32-bit system process address space is limited by 32 bits, so it will fail. On 64-bit you'll have a lot of process address space allocated.

what if I often allocate / deallocate these several 1000's blocks?

You'll bother glibc allocator.