修改 brk 的实现方式，将内存块的分配方式从首次适配更改为最佳适配
alloc.c
修改 /usr/src/servers/pm/alloc.c 中的 alloc_mem 函数，把首次适配修改成改为最佳适配。
即分配内存时，遍历整个空闲内存块列表，找到最适合大小的空闲块
PUBLIC phys_clicks alloc_mem(clicks)phys_clicks clicks; /* amount of memory requested */{ /* Allocate a block of memory from the free list using best fit. The block * consists of a sequence of contiguous bytes, whose length in clicks is * given by 'clicks'. A pointer to the block is returned. The block is * always on a click boundary. This procedure is called when memory is * needed for FORK or EXEC. Swap other processes out if needed. */ register struct hole* hp, * prev_ptr, * best_ptr, * temp; phys_clicks old_base; int flag = 0; do { prev_ptr = NIL_HOLE; hp = hole_head; while (hp != NIL_HOLE && hp->h_base < swap_base) { if (hp->h_len >= clicks && ((hp->h_len < best_ptr->h_len) || (flag == 0))) { best_ptr = hp; temp = prev_ptr; flag = 1; } prev_ptr = hp; hp = hp->h_next; } } while (swap_out()); /* try to swap some other process out */ if (flag == 1) { old_base = best_ptr->h_base; best_ptr->h_base += clicks; best_ptr->h_len -= clicks; /* Remember new high watermark of used memory. */ if (best_ptr->h_base > high_watermark) high_watermark = best_ptr->h_base; /* Delete the hole if used up completely. */ if (best_ptr->h_len == 0) del_slot(temp, best_ptr); /* Return the start address of the acquired block. */ return(old_base); } return(NO_MEM);}修改 brk 中的 adjust 函数
修改 adjust 函数，计算程序当前的空闲空间是否足够分配：若足够，则调整数据段指针，堆栈指针；若不够，调用 allocate_new_mem 函数申请新的足够大的内存空间
/* Compute size of gap between stack and data segments. */ delta = (long) mem_sp->mem_vir - (long) sp_click; lower = (delta > 0 ? sp_click : mem_sp->mem_vir);#define SAFETY_BYTES (384 * sizeof(char *))#define SAFETY_CLICKS ((SAFETY_BYTES + CLICK_SIZE - 1) / CLICK_SIZE) gap_base = mem_dp->mem_vir + data_clicks + SAFETY_CLICKS; if (lower < gap_base){ /* data and stack collided */ if(allocate_new_mem(rmp, (phys_clicks)(mem_sp->mem_vir + mem_sp->mem_len- mem_dp->mem_vir)) == ENOMEM) return(ENOMEM);}被调用的 allocate_new_mem 函数要完成三件事情：
- 申请一段足够大的内存空间：
alloc_mem(new_mem_clicks) - 将程序现有的数据段和堆栈段的内容分别拷贝至新内存区：
sys_abscopy(old_data_addr, new_data_addr, databytes) - 释放原空间：
free_mem(old_data_base, old_mem_clicks)
PUBLIC int allocate_new_mem(rmp, old_mem_clicks)register struct mproc* rmp;phys_clicks old_mem_clicks;{ register struct mem_map* mem_sp, * mem_dp; phys_clicks new_mem_clicks, new_data_base, old_data_base, old_stack_base, new_stack_base; phys_bytes new_data_addr, old_data_addr, new_stack_addr, old_stack_addr; phys_bytes databytes, stackbytes; mem_dp = &rmp->mp_seg[D]; mem_sp = &rmp->mp_seg[S]; new_mem_clicks = old_mem_clicks * 2; new_data_base = alloc_mem(new_mem_clicks); if (new_data_base == NO_MEM) return (NO_MEM); old_data_base = mem_dp->mem_phys, old_stack_base = mem_sp->mem_phys; new_stack_base = new_data_base + new_mem_clicks - mem_sp->mem_len; new_data_addr = (phys_bytes)(new_data_base << CLICK_SHIFT); old_data_addr = (phys_bytes)(old_data_base << CLICK_SHIFT); new_stack_addr = (phys_bytes)(new_stack_base << CLICK_SHIFT); old_stack_addr = (phys_bytes)(old_stack_base << CLICK_SHIFT); databytes = (phys_bytes)(mem_dp->mem_len << CLICK_SHIFT); stackbytes = (phys_bytes)(mem_sp->mem_len << CLICK_SHIFT); if (sys_abscopy(old_data_addr, new_data_addr, databytes) < 0 || sys_abscopy(old_stack_addr, new_stack_addr, stackbytes) < 0) panic(__FILE__, "do_fork can't copy", -1); rmp->mp_seg[D].mem_phys = new_data_base; rmp->mp_seg[S].mem_phys = new_stack_base; rmp->mp_seg[S].mem_vir = mem_dp->mem_vir + new_mem_clicks - mem_sp - > mem_len; free_mem(old_data_base, old_mem_clicks); return (OK);}测试结果
