Build your own Linux

Re:Build your own Linux by JoshTriplett on Wednesday August 16, 2017 @04:19PM (#15031006)

> And I believe all the major 64-bit instruction sets actually use 48-bit addresses at present, so software/compilers can use the top 16 as tag bits.

Not at all a safe assumption in general; architectures evolve over time, and in addition, that address space is split such that kernel addresses have those bits set, not cleared.

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Re:Build your own Linux by smitherfield on Wednesday August 16, 2017 @05:26PM (#15031515)

Didn't mean to imply that was a good idea, just that it's possible. It looks like the top supercomputers are getting close to surpassing 64 petabytes (56 bits byte-addressed) of virtual memory space, so 16-bit tags are right out. A 48-bit byte-addressed memory space is 256 terabytes, which is getting snug for high-end servers but still 15±5 years out for consumer hardware.

  48-bit byte-addressed: 256 terabytes
  48-bit word-addressed:   2 petabytes
  56-bit byte-addressed:  64 petabytes
  56-bit word-addressed: 512 petabytes

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Re:Build your own Linux by justincormack on Thursday August 17, 2017 @05:18AM (#15035036)

NVDIMMs are about to change this, addressed like memory but with around 10 times as much data, so address space is about to jump.

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