This post is going to be little vague. As I will not be explaining all the basics which most of the book of the same topic covers.
Lets talk about kernel memory allocation Most of the time we use kmalloc() function. Although it is supposed to be used only when kernel need contiguous physical memory. But to save overhead caused by vmalloc(), most of the time kmalloc() is preffered.
So How it allocates ?
Before that first we should understand the hierarchy, on top kmalloc() call is there, then slab allocator comes and finally buddy allocator which finally interacts with the physical memory. So at the time of boot up, slab allocator asks buddy allocator to return a fixed no of pages. Further slab allocator will divide those pages in different size of Caches(like 32, 64 ...MAX byte).
Now suppose in your kernel code you want to allocate a memory for int using kmalloc(), it will search the position for this data only in 32 bit slab. Further every slab has 3 lists, full, partial and empty. These list will have pages. So whenever it wants to allocate a memory for any object, it will first check if any partial page available for that slab, if yes it will allocate memory in that page, if not it will check for free pages, if there also no page is available it will ask buddy allocator to give new page.
Use of High Memory:
It is used when you have more physical memory than required virtual memory. For example, for 4GB virtual memory and 1.5 GB RAM, you will have 1 GB for kernel and 3GB for user space. So you have more physical memory (1.5 GB) than virtual memory(1 GB).
So there is no one to one mapping exists between physical and virtual memory, to solve this problem, in x86 architecture, they created one-to-one mapping for upto 896 MB, memory more than that will be dynamically mapped, so for high memory page will have NULL as a virtual address in page structure.
Lets talk about kernel memory allocation Most of the time we use kmalloc() function. Although it is supposed to be used only when kernel need contiguous physical memory. But to save overhead caused by vmalloc(), most of the time kmalloc() is preffered.
So How it allocates ?
Before that first we should understand the hierarchy, on top kmalloc() call is there, then slab allocator comes and finally buddy allocator which finally interacts with the physical memory. So at the time of boot up, slab allocator asks buddy allocator to return a fixed no of pages. Further slab allocator will divide those pages in different size of Caches(like 32, 64 ...MAX byte).
Now suppose in your kernel code you want to allocate a memory for int using kmalloc(), it will search the position for this data only in 32 bit slab. Further every slab has 3 lists, full, partial and empty. These list will have pages. So whenever it wants to allocate a memory for any object, it will first check if any partial page available for that slab, if yes it will allocate memory in that page, if not it will check for free pages, if there also no page is available it will ask buddy allocator to give new page.
Use of High Memory:
It is used when you have more physical memory than required virtual memory. For example, for 4GB virtual memory and 1.5 GB RAM, you will have 1 GB for kernel and 3GB for user space. So you have more physical memory (1.5 GB) than virtual memory(1 GB).
So there is no one to one mapping exists between physical and virtual memory, to solve this problem, in x86 architecture, they created one-to-one mapping for upto 896 MB, memory more than that will be dynamically mapped, so for high memory page will have NULL as a virtual address in page structure.
No comments:
Post a Comment