Comment by wat10000
7 hours ago
Turning these numbers into "memory consumption" gets complicated to the point of being intractable.
The portions that are allocated but not yet used might just be page table entries with no backing memory, making them free. Except for the memory tracking the page table entries. Almost free....
A lot of "image" will be mmapped and clean. Anything you don't actually use from that will be similarly freeish. Anything that's constantly needed will use memory. Except if it's mapped into multiple processes, then it's needed but responsibility is spread out. How do you count an app's memory usage when there's a big chunk of code that needs to sit in RAM as long as any of a dozen processes are running? How do you count code that might be used sometime in the next few minutes or might not be depending on what the user does?
This assumes that executable code pages can be shared between processes. I'm skeptical that this is still a notable optimization on modern systems because dynamic linking writes to executable memory to perform relocations in the loaded code. So this would counteract copy on write. And at least with ASLR, the result should be different for each process anyway.
ld writes to the GOT. The executable segment where .text lives is not written to (it's position independent code in dynamic libraries).
ASLR is not an obstacle -- the same exact code can be mapped into different base addresses in different processes, so they can be backed by the same actual memory.
Dynamic linking doesn't have to write to code. I'm not familiar with other platforms, but on macOS, relocations are all in data, and any code that needs a relocation will indirect through non-code pages. I assume it's similar on other OSes.
This optimization is essential. A typical process maps in hundreds of megabytes of code from the OS. There are hundreds of processes running at any given time. Eyeballing the numbers on an older Mac I have here (a newer one would surely be worse) I'd need maybe 50GB of RAM just to hold the code of all the running processes if the pages couldn't be shared.