为什么共享内存（在ipc中）不需要上下文切换？ 它是从内核空间映射到用户空间的内存吗？

Question

I want to understand if shared memory gets memory allocation from kernel space then why it is not going through context switching ? And if it is not from kernel space then from where this memory gets allocated.

Answer 1

In most modern computers memory isn't allocated from kernel space. Rather the kernel finds a page of physical memory and then maps it into a process at a virtual address that the process is not currently using. The physical address and the virtual address in the process are not the same. So the memory is always "user space" memory. This is all part of the Virtual Memory subsystem.

To share the physical page between processes the kernel maps the page into both processes. Usually at the same virtual address in both. Once this is done the kernel is no longer involved as both processes have the same physical memory mapped at that location. Thus any change will show up for both.

Note: Kernel memory is memory usually only accessible to the kernel and is a different concept.

Answer 2

When shmget() call is made with a requirement of some memory then it makes context switching from user to kernel space, system call service routine runs in kernel with the arguments passed from the user space to revert back with the required memory space [and this memory page is not a part of kernel space its just that its not yet mapped into process memory yet] which gets mapped to process local address space

So this means there is a reserved memory in memory management which is not a part of kernel memory nor it is mapped in process local address space and this memory is used to facilitate such request.