操作系统教程09.ppt

Exercises 9.4 9.5 9.14 End of Chapter 9 Buddy System Allocator 4 a 21KB request Slab Allocator 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. E.g., semaphores, file objects When cache created, filled with objects marked as free When structures stored, objects marked as used If slab 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 Issues -- Prepaging Prepaging To reduce the no. of page faults at process startup Prepage all/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 pages? α near zero ? prepaging loses Other Issues – TLB Reach TLB Reach - The amount of memory accessible from the TLB TLB Reach = (TLB Size) X (Page Size) Ideally, the working set of each process is stored in the TLB Otherwise there is a high degree of page faults Increase the Page Size This may lead to an increase in fragmentation as not all applications require a large page size Provide Multiple Page Sizes This allows applications that require larger page sizes the opportunity to use them without an increase in fragmentation Other Issues – Program Structure Program structure Int[128,128] data; Each row is stored in one page Program 1 for (j = 0; j 128; j++) for (i = 0; i 128; i++) data[i,j] = 0; 128 x 128 = 16,384 page faults  Program 2 for (i = 0; i 128; i++) for (j = 0; j 128; j++) data[i,j] = 0; 128 page faults Other Issues – I/O interlock I/O Inte