From: david.brown@hesbynett.no   
      
   On 22/09/2025 17:28, Stefan Monnier wrote:   
   >> But, AFAIK the ARM cores tend to use significantly less power when   
   >> emulating x86 than a typical Intel or AMD CPU, even if slower.   
   >   
   > AFAIK datacenters still use a lot of x86 CPUs, even though most of them   
   > run software that's just as easily available for ARM.  And many   
   > datacenters care more about "perf per watt" than raw performance.   
   >   
   > So, I think the difference in power consumption does not favor ARM   
   > nearly as significantly as you think.   
   >   
      
   Yes, I think that is correct.   
      
   A lot of it, as far as I have read, comes down to the type of   
   calculation you are doing.  ARM cores can often be a lot more efficient   
   at general integer work and other common actions, as a result of a   
   better designed instruction set and register set.  But once you are   
   using slightly more specific hardware features - vector processing,   
   floating point, acceleration for cryptography, etc., it's all much the   
   same.  It takes roughly the same energy to do these things regardless of   
   the instruction set.  Cache memory takes about the same power, as do PCI   
   interfaces, memory interfaces, and everything else that takes up power   
   on a chip.   
      
   So when you have a relatively small device - such as what you need for a   
   mobile phone - the instruction set and architecture makes a significant   
   difference and ARM is a lot more power-efficient than x86.  (If you go   
   smaller - small embedded systems - x86 is totally non-existent because   
   an x86 microcontroller would be an order of magnitude bigger, more   
   expensive and power-consuming than an ARM core.)  But when you have big   
   processors for servers, and are using a significant fraction of the   
   processor's computing power, the details of the core matter a lot less.   
      
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