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server.lruclock = getLRUClock();
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getLRUClock函数如下:
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#define LRU_CLOCK_RESOLUTION 1000 /* LRU clock resolution in ms */
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#define LRU_BITS 24
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#define LRU_CLOCK_MAX ((1<lru */
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/* Return the LRU clock, based on the clock resolution. This is a time
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* in a reduced-bits format that can be used to set and check the
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* object->lru field of redisObject structures. */
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unsigned int getLRUClock(void) {
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return (mstime()/LRU_CLOCK_RESOLUTION) & LRU_CLOCK_MAX;
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}
因此lrulock最大能到(2**24-1)/3600/24 = 194天,如果超过了这个时间，lrulock重新开始。
对于redis server来说，server.lrulock表示的是一个全局的lrulock，那么对于每个redisObject都有一个自己的lrulock。这样每redisObject就可以根据自己的lrulock和全局的server.lrulock比较，来确定是否能够被淘汰掉。
redis key对应的value的存放对象:
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typedef struct redisObject {
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unsigned type:4;
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unsigned encoding:4;
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unsigned lru:LRU_BITS; /* LRU time (relative to server.lruclock) or
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* LFU data (least significant 8 bits frequency
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* and most significant 16 bits decreas time). */
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int refcount;
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void *ptr;
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} robj
那么什么时候，lru会被更新呢 ？访问该key，lru都会被更新，这样该key就能及时的被移动到lru头部，从而避免从lru中淘汰。下面是这一部分的实现:
点击(此处)折叠或打开
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/* Low level key lookup API, not actually called directly from commands
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* implementations that should instead rely on lookupKeyRead(),
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* lookupKeyWrite() and lookupKeyReadWithFlags(). */
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robj *lookupKey(redisDb *db, robj *key, int flags) {
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dictEntry *de = dictFind(db->dict,key->ptr);
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if (de) {
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robj *val = dictGetVal(de);
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/* Update the access time for the ageing algorithm.
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* Don't do it if we have a saving child, as this will trigger
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* a copy on write madness. */
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if (server.rdb_child_pid == -1 &&
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server.aof_child_pid == -1 &&
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!(flags & LOOKUP_NOTOUCH))
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{
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if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
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unsigned long ldt = val->lru >> 8;
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unsigned long counter = LFULogIncr(val->lru & 255);
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val->lru = (ldt << 8) | counter;
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} else {
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val->lru = LRU_CLOCK();
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}
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}
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return val;
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} else {
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return NULL;
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}
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}
接下来，我们在来分析，key的lru淘汰策略如何实现，分别有哪几种:
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