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What is the maximum supportable RAM by Linux? Assume that hardware is/supports 64-bit. Among all Linux distros.

Does it go up to 16 exabytes, or is it limited like with Windows which I believe is 192 gigabytes?

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4  
Not sure, but note that mainstream 64-bit CPUs (AMD64) only support a 48-bit address space, meaning 256 TB. X86-64 Why do 64 bit systems only have a 48 bit address space –  Mikel Feb 24 at 5:03
    
Your data on Windows is out of date, they can go up to 4T depending on edition. –  Mat Feb 24 at 5:33
    
8 upvotes already? I just turn around for a while and impossible things are happening. –  Rik Telner Feb 24 at 11:25
    
@Mat 192GB was Windows 7. 256GB is Windows 8. 512GB the special ones. None of them goes over 1TB, surely not up to 4TB. didyouknow.org/maximum-ram-on-windows-7-and-windows-8 –  Rik Telner Feb 28 at 17:04
    

1 Answer 1

up vote 14 down vote accepted

Red Hat Enterprise Linux (RHEL)

These are probably a good basis, looking at RHEL6's capabilities, they're covered here, titled: Red Hat Enterprise Linux 6 technology capabilities and limits.

   ss #1

NOTE: [5] The architectural limits are based on the capabilities of the Red Hat Enterprise Linux kernel and the physical hardware. Red Hat Enterprise Linux 6 limit is based on 46-bit physical memory addressing. Red Hat Enterprise Linux 5 limit is based on 40-bit physical memory addressing. All system memory should be balanced across NUMA nodes in a NUMA-capable system.

Kernel docs

Also if you take a look at the kernel docs, Documentation/x86/x86_64/mm.txt:

Virtual memory map with 4 level page tables:

0000000000000000 - 00007fffffffffff (=47 bits) user space, different per mm

So 247 bytes = 128TiB

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128TB RAM, imagine that power on your machine. "Dayum!" –  Rik Telner Feb 24 at 11:26
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@RikTelner that much power on your machine would be absolutely useless. Adding RAM only helps until you reach the ceiling above which you simply never use the extra RAM. Nothing you are likely to run on a home computer will even come close to using 128TB of RAM. –  terdon Feb 24 at 12:46
    
@terdon - this is for servers. We would regularly have VM servers (Massive Dell 910 boxes running VMware) that would come stock with 128/256/512GB RAM. –  slm Feb 24 at 13:34
    
Servers are another matter altogether, my point was that RAM is not a magic wand that speeds up your computer with no limit. It only makes a difference if your processes actually use it and terabytes of RAM will not be used today except in very specific and unusual cases. Having a few teras of RAM on my laptop will not make it work any faster for my daily use having, say 16G or so. And anyway, 512G is a far cry from 128T! –  terdon Feb 24 at 13:39
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@RikTelner: We will see machines with more than 48 bit in 2020 (latest). Those in-memory-dbs want a lot of memory. –  Martin Schröder Feb 26 at 8:53

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