Fractured Memory Pages during Memory Allocation?

Question

My current situation is the following: I am allocating tbytes with malloc(t) then I fill this memory with data (just to force the mapping from virtual to physical memory). The memory page size is 4kB.

Given this situation, I have the following questions:

1.) How much physical memory is used by this (I see the following options):

• exactly tbytes no wasted memory (not even a partial memory page)
• tB + at most 4kB (one memory page of waste it can be used within the same process).
• tB + at most 4kB (one memory page of waste it cannot be used while the allocation exists)
• potentially more than one page of waste due to the mapping technique (some power of 2 magic or blocks of pages... other reasons)

2.) Will this continous piece of virtual memory (one allocation) also be mapped to a continous piece of physical memory?

3.) If there is some waste, can I measure the exact memory footprint of my application? preferably from inside C++.

I should note, that currently my code runs on a ubuntu linux machine, but insights into other systems would still be welcome.

Answer 1

1. • First of all the memory allocations are always rounded (up) to page size.
   • I didn't understand what you said "while allocation exists"...
   • Although it can cause many problems, every thread in your process who has a pointer to the allocated memory can access it (so you need to synchronize those threads etc.
2. You have already got an answer.
3. You can use mmap