CODEWITHSUNDEEP
linux-kernelvirtualizationintel
Shivendra Mishra
Shivendra Mishra

Reputation: 688

Why do we have memory zones in linux?

I was reading this on a page that:

Because of hardware limitations, the kernel cannot treat all pages as identical. Some pages, because of their physical address in memory, cannot be used for certain tasks. Because of this limitation, the kernel divides pages into different zones.

I was wondering about those hardware limitation. Can somebody please explain me those hardware limitation and give an example. As well, is there any software guide from intel explaining this?

Also, I read that virtual memory is divided into two parts 1GB for kernel space and 3GB for user space. Why do we give 1GB space in the virtual space of all processes to kernel? How is it mapped to actual physical pages? Can somebody please point me to a clean text explaining this?

Thanks in advance.

Upvotes: 0

Views: 834

Answers (2)

incompetent
incompetent

Reputation: 1822

for your second question about 1GB kernel mapping in user space for a processor

kernel is mapped of course for time saving by not having switch. 1 GB is for kernel functionality so that if kernel maps new memory for its functionality kernel can do that. any book on Unix can give you details

Upvotes: 1

Nidhoegger
Nidhoegger

Reputation: 5252

The hardware limitations mostly concern old devies. For example, you have the ZONE_DMA, which is from 0 - 16MB. This is e.g. needed for older ISA Devices, which are not capable of adressing above the 16MB limit. Then you have the ZONE_NORMAL, where most of the kernel operations take place and is adressed permanently into the kernels adress space.

The 1GB and 3GB split is simple. You have virtual adresses here, so for your application, the memory adress always starts at 0x00000000, reserved are the 1st GB of this for kernel stuff. Why this is done is pretty simple: You have the kernel mode and the user mode. In kernel mode you are allowed to use system calls. If you would not have the kernel memory mapped to your virtual adress space, you would have to do a context switch to trap you into kernel mode (context switch: Save current context to memory, load another context from memory -> time consuming). But as kernel-mode operations can take place in the same virtual adress space, you dont need to switch the context to, for example, allocate new memory or do any other system call.

Upvotes: 2

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