The downside to tools like UPX is that the executable code is actively transformed on load. This limits the ability to use shared memory for multiple concurrent executions of the same executable.
If the OS loads executables by mmap and load on page-hit, you can potentially save memory by not ever loading unused parts of an executable. a transform-on-load requires the entire program to be loaded before execution begins.
'... never loading unused parts of an executable...' - this is one of the benefits of a paging system. Paging was (is?) the best way to keep memory requirement down. The way Multics worked was by having paged segments. http://www.osinfoblog.com/post/136/segmentation-with-paging:... The x86(-64) can support something like this, but as far as I know, no modern OS supports this feature.
Linux does this, as does basically every other modern OS.
Pick a random large process on a linux host - like your web browser. cat /proc/$pid/status.
VmExe is the size of the mapped executable; VmLib is the size of all the other mapped libraries and executable pages. Add those two numbers together to find the size of all the executable code mapped into this binary.
VmRSS is the amount of physical memory that the process is currently using. You'll find that this is a lot smaller than the code mapped into the binary. That's because the kernel hasn't loaded any of that into physical memory.
Thank you. I'm not at all sure why the 'old hands' here on HN have decided to so viciously downvote this. I didn't call anybody names, didn't violate any posting rules, did not violate etiquette, and provided a very nice link to Multics.
If there is a perceived error in what I wrote, then, like the one nice responder, explain, please.
HN is such a different community now than when I joined 1,835 days ago. It brings tears to my eyes.
If the OS loads executables by mmap and load on page-hit, you can potentially save memory by not ever loading unused parts of an executable. a transform-on-load requires the entire program to be loaded before execution begins.