How os handle fragmentation in virtual address space

Question

据我所知，分页系统确实消除了物理地址空间中的外部碎片，但是虚拟地址空间中的碎片又如何呢？

Answer 1

In modern OSes the virtual address space is used per process (the kernel has it's own dedicated virtual range), which means that the demands are much lower compared to the whole OS. The virtual address space is usually large enough (2-3 GB per process on x86 and multiple TB (8 on Windows) on x64 machines), so that fragmentation is not such a big issue as for the OS-wide physical address space. Still the issue can arise, especially for long running and memory hungry applications on x86 or other 32 bit architectures. For this the OS provides mechanisms, for example in form of the heap code. An application usually reserves one or more memory ranges as heap(s) when it starts and allocates the required chunks of memory from there later (eg malloc). There are a varity of implementations that handle fragmentation of the heap in different ways. Windows provides a special low-fragmentation heap implementation that can be used, if desired. Everything else is usually up to the application or it's libraries.

Answer 2

Let me add a qualification to your statement. Paging systems nearly eliminate fragmentation in the physical address space when the kernel is pageable.

On some systems, the user mode page tables are themselves pageable. On others, they are are physical locations that are not pageable. Then you can get fragmentation.

Fragmentation in the virtual address space tends to occur in heap allocation. The challenge of heap managers is to manage the space while minimizing fragmentation.