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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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.
NOTE:  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.
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