[理学]中山大学软件学院操作系统概念上课课件第9章.ppt

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Allocating Kernel Memory Treated differently from user memory Often allocated from a free-memory pool Kernel requests memory for structures of varying sizes Some kernel memory needs to be contiguous I.e. for device I/O 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 For example, assume 256KB chunk available, kernel requests 21KB Split into AL and Ar of 128KB each One further divided into BL and BR of 64KB One further into CL and CR of 32KB each – one used to satisfy request Advantage – quickly coalesce unused chunks into larger chunk Disadvantage - fragmentation Buddy System Allocator 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 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 Slab Allocation Other Considerations -- Prepaging Prepaging To reduce the large number of page faults that occurs at process startup Prepage all or some of the pages a process will need, before they are referenced But if prepaged pages are unused, I/O and memory was wasted Assume s pages are prepaged and α of the pages is used Is cost of s * α save pages faults or than the cost of prepaging s * (1- α) unnecessary pa