4 ms·

Aren't 64-bit processors a little more efficient at running a 64-bit operating system?

by udp 15y ago

CurtHagenlocher 15y ago
No. And for most programs, 64-bit code is probably less efficient than 32-bit code. Pointers are twice as big and instructions are generally longer, so less stuff fits into cache.
- stephanimal 15y ago
  This is one disadvantage, yes. However all registers are now 64 bit, and in general, there are more of them. Furthermore, the 4GB limit does not just concern physical RAM, it also applies elsewhere, like memory mapped files. In general, it is unlikely that you will see poorer performance from 64-bit code compared to its 32-bit counterpart.
  - herge 15y ago
    Most of the memory used by programs are pointers, and if you double your pointer size, it has a negative impact on the efficientcy of the L1 and L2 cache.
    - stephanimal 15y ago
      Yes, this is exactly what CurtHagenlocher said. >> Most of the memory used by programs are pointers This is a huge generalization, may I add. Yes, I will not argue that cache usage is less efficient, but I am sure you know that register access is faster than cache access, and we double the amount of registers in x64.
      - herge 15y ago
        To quote Donald Knuth, a 64 bit program would "waste half the memory [and] effectively throw away half of the cache". You may double the number of registers, but you are still shuffling tons of data between your caches and memory, and that is going to be your constraint for most programs. Losing half of the 4mb or 12mb of your L2 or L3 cache because you use 64 bit pointers has a much bigger impact than doubling the number of registers.
- Symmetry 15y ago
  For almost every architecture out there the transition to 64-bits made existing code run more slowly for the reasons you describe, but on x86 the 64 bit transition also doubled the number of registers, guaranteed that SSE2 was available, and did some other stuff that actually made most code run faster. However, I believe Google is working on some program to let people use all the goodies in x86-64 while also using 32-bit pointers.

Aren't 64-bit processors a little more efficient at running a 64-bit operating system?

by udp 15y ago

Aren't 64-bit processors a little more efficient at running a 64-bit operating system?

No. And for most programs, 64-bit code is probably less efficient than 32-bit code. Pointers are twice as big and instructions are generally longer, so less stuff fits into cache.

stephanimal 15y ago
This is one disadvantage, yes. However all registers are now 64 bit, and in general, there are more of them. Furthermore, the 4GB limit does not just concern physical RAM, it also applies elsewhere, like memory mapped files. In general, it is unlikely that you will see poorer performance from 64-bit code compared to its 32-bit counterpart.
- herge 15y ago
  Most of the memory used by programs are pointers, and if you double your pointer size, it has a negative impact on the efficientcy of the L1 and L2 cache.
  - stephanimal 15y ago
    Yes, this is exactly what CurtHagenlocher said. >> Most of the memory used by programs are pointers This is a huge generalization, may I add. Yes, I will not argue that cache usage is less efficient, but I am sure you know that register access is faster than cache access, and we double the amount of registers in x64.
    - herge 15y ago
      To quote Donald Knuth, a 64 bit program would "waste half the memory [and] effectively throw away half of the cache". You may double the number of registers, but you are still shuffling tons of data between your caches and memory, and that is going to be your constraint for most programs. Losing half of the 4mb or 12mb of your L2 or L3 cache because you use 64 bit pointers has a much bigger impact than doubling the number of registers.
Symmetry 15y ago
For almost every architecture out there the transition to 64-bits made existing code run more slowly for the reasons you describe, but on x86 the 64 bit transition also doubled the number of registers, guaranteed that SSE2 was available, and did some other stuff that actually made most code run faster. However, I believe Google is working on some program to let people use all the goodies in x86-64 while also using 32-bit pointers.