127.0.0.1:6379> set k1 value1
OK
127.0.0.1:6379> DEBUG OBJECT k1
xx refcount:1 encoding:embstr serializedlength:7 lru:7723189 lru_seconds_idle:160

127.0.0.1:6379> set k1 value1
OK
127.0.0.1:6379> memory  usage k1    //这里k1 value占用57字节内存
(integer) 57
127.0.0.1:6379> memory  usage aaa  // aaa键不存在，返回nil.
(nil)

 /* The memory command will eventually be a complete interface for the
 * memory introspection capabilities of Redis.
 *
 * Usage: MEMORY usage <key> */
void memoryCommand(client *c) {

//函数注释已说明，value计算和取样计算的处理
/* Returns the size in bytes consumed by the key's value in RAM.
 * Note that the returned value is just an approximation, especially in the
 * case of aggregated data types where only "sample_size" elements
 * are checked and averaged to estimate the total size. */
#define OBJ_COMPUTE_SIZE_DEF_SAMPLES 5 /* Default sample size. */  //对于集合类型结构，默认取样个数为5
size_t objectComputeSize(robj *o, size_t sample_size) {

    if (o->type == OBJ_STRING) { //String类型
        --- 省略---------
    } else if (o->type == OBJ_LIST) { //List类型
        if (o->encoding == OBJ_ENCODING_QUICKLIST) {
            quicklist *ql = o->ptr;
            quicklistNode *node = ql->head;
            asize = sizeof(*o)+sizeof(quicklist);
            //获取List的头sample_size个元数，计算长度之和
            do {
                elesize += sizeof(quicklistNode)+ziplistBlobLen(node->zl);
                samples++;
            } while ((node = node->next) && samples < sample_size); 个元素
            asize += (double)elesize/samples*listTypeLength(o);  //求平均 X List元素个数，计算出内存之和
        } ---省略----

127.0.0.1:6379> memory stats
 1) "peak.allocated"
 2) (integer) 3211205544
 3) "total.allocated"
 4) (integer) 875852320
 5) "startup.allocated"
 6) (integer) 765608
 7) "replication.backlog"
 8) (integer) 117440512
 9) "clients.slaves"
10) (integer) 16858
11) "clients.normal"
12) (integer) 49630
13) "aof.buffer"
14) (integer) 0
15) "db.0"
16) 1) "overhead.hashtable.main"
    2) (integer) 48388888
    3) "overhead.hashtable.expires"
    4) (integer) 104
17) "overhead.total"
18) (integer) 166661600
19) "keys.count"
20) (integer) 1000007
21) "keys.bytes-per-key"
22) (integer) 875
23) "dataset.bytes"
24) (integer) 709190720
25) "dataset.percentage"
26) "81.042335510253906"
27) "peak.percentage"
28) "27.274873733520508"
29) "fragmentation"
30) "0.90553224086761475"

127.0.0.1:6379> memory doctor
"Sam, I detected a few issues in this Redis instance memory implants:\n\n
Peak memory: In the past this instance used more than 150% the memory that is currently using. 
The allocator is normally not able to release memory after a peak, 
so you can expect to see a big fragmentation ratio,
however this is actually harmless and is only due to the memory peak, and if the Redis instance Resident Set Size (RSS) is currently bigger than expected,
the memory will be used as soon as you fill the Redis instance with more data.
 If the memory peak was only occasional and you want to try to reclaim memory, 
 please try the MEMORY PURGE command, otherwise the only other option is to 
 shutdown and restart the instance.\n\n
 I'm here to keep you safe, Sam. I want to help you.\n"

sds getMemoryDoctorReport(void) {
    int empty = 0;          /* Instance is empty or almost empty. */
    int big_peak = 0;       /* Memory peak is much larger than used mem. */
    int high_frag = 0;      /* High fragmentation. */
    int big_slave_buf = 0;  /* Slave buffers are too big. */
    int big_client_buf = 0; /* Client buffers are too big. */
    int num_reports = 0;
    struct redisMemOverhead *mh = getMemoryOverheadData();  //获取各个内存指标，用于后面进行条件判断

    if (mh->total_allocated < (1024*1024*5)) {  // 如果使用内存小于5MB,则判断几乎是空实例，不进行其他诊断
        empty = 1;
        num_reports++;
    } else {
        ---- 省略---
        /* Fragmentation is higher than 1.4? */
        if (mh->fragmentation > 1.4) {
            high_frag = 1;
            num_reports++;
        }


        /* Slaves using more than 10 MB each? */
        if (numslaves > 0 && mh->clients_slaves / numslaves > (1024*1024*10)) {
            big_slave_buf = 1;
            num_reports++;
        }
    }

    sds s;
    if (num_reports == 0) {
        s = sdsnew(
        "Hi Sam, I can't find any memory issue in your instance. "
        "I can only account for what occurs on this base.\n");
    } else if (empty == 1) {
        s = sdsnew(
        "Hi Sam, this instance is empty or is using very little memory, "
        "my issues detector can't be used in these conditions. "
        "Please, leave for your mission on Earth and fill it with some data. "
        "The new Sam and I will be back to our programming as soon as I "
        "finished rebooting.\n");
    } else {
        if (high_frag) {
            s = sdscatprintf(s," * High fragmentation: This instance has a memory fragmentation greater than 1.4 (this means that the Resident Set Size of the Redis process is much larger than the sum of the logical allocations Redis performed). This problem is usually due either to a large peak memory (check if there is a peak memory entry above in the report) or may result from a workload that causes the allocator to fragment memory a lot. If the problem is a large peak memory, then there is no issue. Otherwise, make sure you are using the Jemalloc allocator and not the default libc malloc. Note: The currently used allocator is \"%s\".\n\n", ZMALLOC_LIB);
        }
        if (big_slave_buf) {
            s = sdscat(s," * Big slave buffers: The slave output buffers in this instance are greater than 10MB for each slave (on average). This likely means that there is some slave instance that is struggling receiving data, either because it is too slow or because of networking issues. As a result, data piles on the master output buffers. Please try to identify what slave is not receiving data correctly and why. You can use the INFO output in order to check the slaves delays and the CLIENT LIST command to check the output buffers of each slave.\n\n");
        }
        if (big_client_buf) {

}

127.0.0.1:6379> info memory
# Memory
used_memory:344944360
used_memory_human:328.96M
used_memory_rss:2828042240
used_memory_rss_human:2.63G
mem_fragmentation_ratio:8.20
mem_allocator:jemalloc-4.0.3
active_defrag_running:0
lazyfree_pending_objects:0
127.0.0.1:6379> memory purge
OK
127.0.0.1:6379> info memory
# Memory
used_memory:344942912
used_memory_human:328.96M
used_memory_rss:2522521600
used_memory_rss_human:2.35G
used_memory_dataset_perc:86.02%
mem_fragmentation_ratio:7.31
mem_allocator:jemalloc-4.0.3
active_defrag_running:0
lazyfree_pending_objects:0

else if (!strcasecmp(c->argv[1]->ptr,"purge") && c->argc == 2) {
#if defined(USE_JEMALLOC)  //判断当前redis使用提malloc是否为jemalloc
        char tmp[32];
        unsigned narenas = 0;
        size_t sz = sizeof(unsigned);
        if (!je_mallctl("arenas.narenas", &narenas, &sz, NULL, 0)) { //调用jemalloc的处理
            sprintf(tmp, "arena.%d.purge", narenas);
            if (!je_mallctl(tmp, NULL, 0, NULL, 0)) {
                addReply(c, shared.ok);
                return;
            }
        }
        addReplyError(c, "Error purging dirty pages");
#else
        addReply(c, shared.ok);
        /* Nothing to do for other allocators. */
#endif

127.0.0.1:6379> memory malloc-stats
___ Begin jemalloc statistics ___
Version: 4.0.3-0-ge9192eacf8935e29fc62fddc2701f7942b1cc02c
Assertions disabled
Run-time option settings:
  opt.abort: false
  opt.lg_chunk: 21
  opt.dss: "secondary"
  opt.narenas: 96
  opt.lg_dirty_mult: 3 (arenas.lg_dirty_mult: 3)
  opt.stats_print: false
  opt.junk: "false"
  opt.quarantine: 0
  opt.redzone: false
  opt.zero: false
  opt.tcache: true
  opt.lg_tcache_max: 15
CPUs: 24
Arenas: 96
Pointer size: 8
Quantum size: 8
Page size: 4096
Min active:dirty page ratio per arena: 8:1
Maximum thread-cached size class: 32768
Chunk size: 2097152 (2^21)
Allocated: 345935320, active: 350318592, metadata: 65191296, resident: 455610368, mapped: 2501902336
Current active ceiling: 352321536

arenas[0]:
assigned threads: 1
dss allocation precedence: secondary
min active:dirty page ratio: 8:1
dirty pages: 85527:10020 active:dirty, 82084 sweeps, 112369 madvises, 665894 purged
                            allocated      nmalloc      ndalloc    nrequests
small:                      311397848     12825077     11674091     32248603
large:                         983040         1850         1842         1854
huge:                        33554432            8            6            8
total:                      345935320     12826935     11675939     32250465
--省略------