innodb buffer pool管理--LRU调整

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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;        }    }

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