raddevus wrote:
I am still very curious if a 64 bit app can eat all of the memory on a large server (64GB RAM or something larger). I'm guessing that it cannot since 1. I believe that any app cannot allocate RAM beyond its address space.
If you by 'address space' refer to the entire 2**64 bytes space that a 64 bit process can cover by its addresses, 16 exbi bytes (more than 16 million gigabytes), your assumption is right: A process cannot allocate that much space. And we will never see a computer with 16 exbi bytes of RAM. Never ever. In no general machine (excluding e.g. embedded processors) of the last 30-40 years has the address indicated by the program code been used directly as the physical RAM address. The virtual address in the program is translated to a different physical address in RAM through a set of hardware translation tables, managed by the OS, called the Memory Management System (MMS). Each process has its own set of MMS tables. The OS sets up the MMS tables for a tiny slice of the virtual address space. If the program presents a virtual address within this slice, the range covered by the MMS tables for that process, it is translated to a physical RAM address. If the virtual address is outside the range covered by the MMS tables, an interrupt is generated, and the OS will terminate the process. (Well, it might offer a mechanism for reporting the interrupt e.g. to a debugger that can inspect the process state before it is cleaned out.) If you by 'its address space' refer to just that slice of the total 64 bit virtual address space for which the OS has set up translation tables, then you are essentially right. The size of this slice can be a few hundred kiB, a few GiB, or many GiB - but the OS will not give you more than it is capable of handling. When an app allocates RAM, the allocated space is, at the outset, within the address space translated by its MMS tables. If the malloc/new/... maps down to an OS request, the OS may say: 'There isn't enough unused space in the already mapped virtual address space, so I have to add another entry to the mapping tables, expanding the address space available to that process'. Before the OS does that, it will check that the process does not already control an excessive amount of address space. The limit is set by the OS to any value that it can handle. In many systems, malloc/new/... starts out as a call to a runtime library in the process address space. As long
