innodb buffer pool管理--LRU调整

buf_LRU_old_adjust_len(buf_pool)：

//old_len==512,new_len=min(512*378/1024, 512-25),其实是512的3/8
    // buf_pool->LRU_old_ratio 为378(innodb_old_blocks_pct默认值是37)
    old_len = buf_pool->LRU_old_len;
    new_len = ut_min(UT_LIST_GET_LEN(buf_pool->LRU)
             * buf_pool->LRU_old_ratio / BUF_LRU_OLD_RATIO_DIV,
             UT_LIST_GET_LEN(buf_pool->LRU)- (20+ 5));
    for (;;) {
        buf_page_t*    LRU_old = buf_pool->LRU_old;
        if (old_len +20 < new_len) {
            buf_pool->LRU_old = LRU_old = UT_LIST_GET_PREV(LRU, LRU_old);
            old_len = ++buf_pool->LRU_old_len;
            buf_page_set_old(LRU_old, TRUE);
        }else if (old_len > new_len +20) {
            //512的前209个大概都是young
            buf_pool->LRU_old = UT_LIST_GET_NEXT(LRU, LRU_old);
            old_len = --buf_pool->LRU_old_len;
            buf_page_set_old(LRU_old, FALSE);
        }else{
            Return;
        }
    }