42 bits?

Per Liden per.liden at oracle.com
Fri Mar 15 11:55:55 UTC 2019


The 42 bit limitation we have right now is mostly artificial, in the 
sense that we can fairly easily raise this to 44 bits (16TB), which we 
might do do at some point. Using more than 44 bits is also doable but 
would require more work. The problem, on current x86_64 hardware with 48 
bits of addressable space, is that we're running out of address space to 
play with. Keep in mind that on Linux/x86_64 we do heap multi-mapping, 
which means we need 4x the heap address space.


On 3/15/19 10:43 AM, Simone Bordet wrote:
> Hi,
> while researching about ZGC, I could not find an authoritative source
> that says that Intel CPUs are limited to 42 bits for addressing
> memory, and therefore ZGC uses bits 0-41 for addressing, 42-45 for
> metadata.
> OTOH, I found that there is a somewhat agreement in the search results
> I found (e.g. https://en.wikipedia.org/wiki/X86-64) that 48 bits are
> used for addressing memory.
> Just curious. Thanks for shedding some light on this.

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