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While I am trying to understand the high memory problem for 32-bit cpu and Linux, why is there no high-memory problem for 64-bit cpu?
In particular, how is the division of virtual memory into kernel space and user space changed, so that the requirement of high memory doesn't exist for 64-bit cpu?
Thanks.
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The short answer is yes, 64-bit operating systems almost always require more RAM than corresponding 32-bit operating systems and 64-bit applications often do require a bit more RAM than corresponding 32-bit applications.
The theoretical memory limit that a 64-bit computer can address is about 16 exabytes (16 billion gigabytes), Windows XP x64 is currently limited to 128 GB of physical memory and 8 TB of virtual memory.
Here's why it matters Simply put, a 64-bit processor is more capable than a 32-bit processor because it can handle more data at once. A 64-bit processor can store more computational values, including memory addresses, which means it can access over 4 billion times the physical memory of a 32-bit processor.
In principle, a 64-bit microprocessor can address 16 EiB (16 × 10246 = 264 = 18,446,744,073,709,551,616 bytes, or about 18.4 exabytes) of memory. However, not all instruction sets, and not all processors implementing those instruction sets, support a full 64-bit virtual or physical address space.
A 32-bit system can only address 4GB of memory. In Linux this is divided into 3GB of user space and 1GB of kernel space. This 1GB is sometimes not enough so the kernel might need to map and unmap areas of memory which incurs a fairly significant performance penalty. The kernel space is the "high" 1GB hence the name "high memory problem".
A 64-bit system can address a huge amount of memory - 16 EB -so this issue does not occur there.
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