C# 64 bit applications, large memory address aware

Question

Im a little confused with regards to the memory limitations of an application. As far as i can see, if i write a c# application, targeting x64, my program will have access to 8TB of Virtual address space = space on the HD?

OS >= Windows 7 professional supports 192gigs of RAM. So if i had 192gig system (unfortunately i dont), i could load just over 8.1TB of data into memory (assuming no other processes were running)?

Is virtual memory only used when i have run out available ram? Im assuming there is a performance implication associated with virtual memory vs using RAM?

Apologies if these appear stupid questions, but when it comes to memory management, im rather green.

Answer 1

Your question is actually several related question, taking each individually:

OS >= Windows 7 professional supports 192gigs of RAM. So if i had 192gig system (unfortunately i dont), i could load just over 8.1TB of data into memory (assuming no other processes were running)?

No, it would still be 8 TB. That is the maximum amount of addressable space, whether it is in RAM or elsewhere.

However you could never have 8 TB in use, even if you some how unloaded Windows itself, as the OS needs to keep track of the space being used. In total, you could probably get to 7 TB approximately.

is virtual memory only used when i have run out available ram?

No, if you have virtual memory turned on the entirety of RAM is typically preloaded onto your HDD (give or take a few seconds). This allows the OS to unload something to make room if it feels the need, without having to persist the data. Note that the OS keep thorough track so will know if this is the case or not.

Im assuming there is a performance implication associated with virtual memory vs using RAM?

Depends on your context. Every seek on the hard drive takes a computational eternity, however it is still a fraction of a second. Assuming your process isn't thrashing and repeatedly accessing virtual memory, you should not notice a significant performance hit outside high performance computing.

Apologies if these appear stupid questions, but when it comes to memory management, im rather green.

Your main problem is you have some preconceived notions about how memory works that don't line up with reality. If you are really interested you should look into how memory is used in a modern system.

For instance, most people conceptualize that a pointer points to a location in memory, since it is the fundamental structure. This isn't quite true. In fact the pointer contains a piece of information that can be decoded into a location in the addressable space of the system, which isn't always in RAM. This decoding process uses quite a few tricks that are interesting, but beyond the scope of this question.