The limit of kernel memory in 64bit system

[nick]

Sorry Yiqun,
I was thinking of virtual memory so you are correct. I also haven't looked into
the kernel memory subsystem(s) in a while so I may be a little behind in my knownledge
of them.
Regards Nick 

On 2014-11-29 03:54 PM, Yiqun Chen wrote:
> 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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> 
> 
>