操作系统原理9章节(1023KB).ppt

Allocating Kernel Memory Treated differently from user memory Often allocated from a free-memory pool Kernel requests memory for structures of varying sizes, fragmentation need to be taken care of Some kernel memory needs to be contiguous Operating System Concepts Buddy System Allocates memory from fixed-size segment consisting of physically-contiguous pages Memory allocated using power-of-2 allocator Satisfies requests in units sized as power of 2 Request rounded up to next highest power of 2 When smaller allocation needed than is available, current chunk split into two buddies of next-lower power of 2 Continue until appropriate sized chunk available Operating System Concepts Buddy System Allocator Operating System Concepts 21KB kernel memory allocation request Fragmentation problem Slab Allocator Alternate strategy Slab is one or more physically contiguous pages Cache consists of one or more slabs Single cache for each unique kernel data structure Each cache filled with objects – instantiations of the data structure Operating System Concepts Slab Allocator (Cont) When cache created, filled with objects marked as free When structures stored, objects marked as used If slab is full of used objects, next object allocated from empty slab If no empty slabs, new slab allocated Benefits include no fragmentation, fast memory request satisfaction Operating System Concepts Slab Allocation Operating System Concepts Slab Allocation For example process descriptor is of type struct task_struct Approx 1.7KB of memory New task - allocate new struct from cache Will use existing free struct task_struct Slab can be in three possible states Full – all used Empty – all free Partial – mix of free and used Upon request, slab allocator Uses free struct in partial slab If none, takes one from empty slab If no empty slab, create new empty Operating System Concepts Chapter 9: Virtual Memory Background Demand Paging Copy-on-Write Page Replacement Allocation of Frames Thrashing Memory