The limit of kernel memory in 64bit system

[Yiqun Chen]

Incorrect. The 64 bit machine theoretically supports 2 ^ 64 bytes virtual
memory space but on x86 only 48 out of 64 address buses are used. So we
cannot assume the upper limit of physical space.on different platforms.
And Nick you're confusing with the physical and virtual memory. The usage
of memory for kernel space is not determined by the ratio. It's defined by
mapping strategy. You first need to know why this 3 to 1 was used. It's
because we cannot map all the physical pages onto the corresponding virtual
address since we need to spare some linear space for user program. But on
64 bits platform we have plenty virtual address for the programmer as the
physical memory is much much smaller than it. On x86_64 we divide
kernel/user space by setting all the first 16 bits with 0 or 1 as the user
or kernel space respectfully. So how large would the kernel space be if we
could use 48-bit linear space?

On Sat, Nov 29, 2014 at 8:16 PM, nick <xerofoify at gmail.com> wrote:

> J.Hwan,
> There is a limit as 64 bit can only support 16 extabytes. Greg is right in
> that there is no real limit
> on modern machines because of this. However from my knowledge, system ram
> is split into a 3 to 1 ratio,
> where 75 percent is for user space and the only quarter is for the
> kernel's memory. So on my system with
> 8 GB of ram, the kernel can use up to 25 percent or 2 GB of ram.
> Hope this answers your question,
> Nick
> On 2014-11-29 12:45 PM, Greg KH wrote:
> > On Sat, Nov 29, 2014 at 12:25:43AM +0900, J.Hwan Kim wrote:
> >> Hi, everyone
> >>
> >> As far as I know, the kernel memory limit is 1GB in 32bit system.
> >> Is it also applicable to 64bit system?
> >> What's the limit of kernel memory in 64bit system?
> >
> > There is no limit :)
> >
> > What do you really want to know this for?
> >
> > greg k-h
> >
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-- 
Best regards
Chen, Yiqun
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