一、简述 Memory指令是Redis4.0版本更新的特性，可用于详细的分析内存的使用情况，内存使用诊断，内存碎片回收等工作；
可以通过memory help指令打印出memory指令的信息，详细信息如下所示：
1) MEMORY <subcommand> arg arg ... arg. Subcommands are: 2) DOCTOR - Return memory problems reports. 3) MALLOC-STATS -- Return internal statistics report from the memory allocator. 4) PURGE -- Attempt to purge dirty pages for reclamation by the allocator. 5) STATS -- Return information about the memory usage of the server. 6) USAGE <key> [SAMPLES <count>] -- Return memory in bytes used by <key> and its value. Nested values are sampled up to <count> times (default: 5).
具体的指令相关解释为：
MEMORY DOCKER：返回内存问题报告；MEMORY MALLOC-STATS：从内存分配器返回内部统计信息报告；MEMORY PURGE：尝试通过分配器清除脏页以进行回收；MEMORY STATS：返回有关服务器内存使用情况的信息；MEMORY USAGE key [SAMPLES count]：返回 key 使用的字节数及其值， 嵌套值最多采样 count 次（默认值：5）；
二、具体的指令解析 2.1、MEMORY DOCKER 使用方式：
该指令主要列举条件判断，满足条件的给出检查结果和建议，主要包含以下几点，满足其中一点，就给出诊断结果和建议，检测报告会提示所有检测出的问题，检测结构主要为一下几种情况：
无异常：并没有检测出问题；空实例/内存占用小：示例实际分配内存小于5M，无法进一步进行检测（代码：mh->total_allocated < (1024*1024*5)）；历史内存与当前内存比例过大：redis自启动以来分配的内存峰值/当前的内存大小结果大于 1.5（代码：((float)mh->peak_allocated / mh->total_allocated) > 1.5）；内存碎片率：内存碎片率大于1.4（代码：mh->fragmentation > 1.4）；一般客户端单实例内存：非从库客户端的单实例内存占用大于200KB（代码：mh->clients_normal / numclients > (1024*200)）；从库客户端单实例内存：在有从库的前提下，从库客户端的单实例内存占用大于10M（代码：numslaves > 0 && mh->clients_slaves / numslaves > (1024*1024*10)）；
执行结果示例：
Sam, I detected a few issues in this Redis instance memory implants: * 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 "jemalloc-3.2.0" . I'm here to keep you safe, Sam. I want to help you.
2.2、MEMORY MALLOC-STATS 使用方式：
打印内存分配器状态，只在使用jemalloc时有用；
执行结果示例：
___ Begin jemalloc statistics ___ Version: 3.2.0-0-g87499f6748ebe4817571e817e9f680ccb5bf54a9 Assertions disabled Run-time option settings: opt.abort: false opt.lg_chunk: 22 opt.dss: "secondary" opt.narenas: 128 opt.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: 32 Arenas: 128 Pointer size: 8 Quantum size: 16 Page size: 4096 Min active:dirty page ratio per arena: 8:1 Maximum thread-cached size class: 32768 Chunk size: 4194304 (2^22) Allocated: 5948968, active: 6402048, mapped: 20971520 Current active ceiling: 12582912 chunks: nchunks highchunks curchunks 5 5 5 huge: nmalloc ndalloc allocated 0 0 0 Merged arenas stats: assigned threads: 2 dss allocation precedence: N/A dirty pages: 1563:0 active:dirty, 5 sweeps, 4 madvises, 386 purged allocated nmalloc ndalloc nrequests small: 501288 12362 1272 283989 large: 5447680 12 4 14 total: 5948968 12374 1276 284003 active: 6402048 mapped: 12582912 bins: bin size regs pgs allocated nmalloc ndalloc nrequests nfills nflushes newruns reruns curruns 0 8 501 1 40 109 104 100 3 6 1 0 1 1 16 252 1 164336 10372 101 282735 114 9 41 0 41 2 32 126 1 14432 604 153 751 25 10 5 7 4 3 48 84 1 1200 99 74 181 4 9 1 0 1 4 64 63 1 640 80 70 56 4 6 1 0 1 5 80 50 1 1760 80 58 23 5 6 1 0 1 6 96 84 2 8544 119 30 94 7 7 2 0 2 7 112 72 2 448 80 76 6 3 6 1 0 1 8 128 63 2 768 76 70 19 6 9 1 0 1 [9..10] 11 224 72 4 448 78 76 2 3 6 3 0 1 12 256 63 4 768 72 69 6 4 6 4 0 1 [13..14] 15 448 63 7 28224 63 0 0 1 0 1 0 1 16 512 63 8 1536 69 66 4 3 5 3 0 1 17 640 51 8 0 51 51 1 1 3 1 0 0 18 768 47 9 768 47 46 1 1 3 1 0 1 19 896 45 10 40320 45 0 0 1 0 1 0 1 20 1024 63 16 1024 67 66 4 3 6 2 0 1 [21] 22 1536 42 16 0 47 47 2 2 5 2 0 0 23 1792 38 17 68096 38 0 0 1 0 1 0 1 24 2048 65 33 10240 69 64 2 2 4 1 0 1 25 2560 52 33 17920 58 51 2 2 3 1 0 1 [26] 27 3584 39 35 139776 39 0 0 1 0 1 0 1 large: size pages nmalloc ndalloc nrequests curruns [1] 8192 2 5 3 7 2 [1] 16384 4 2 0 2 2 [3] 32768 8 2 0 2 2 [292] 1232896 301 1 0 1 1 [30] 1359872 332 1 1 1 0 [668] 4100096 1001 1 0 1 1 [17] arenas[0]: assigned threads: 1 dss allocation precedence: disabled dirty pages: 1554:0 active:dirty, 3 sweeps, 4 madvises, 386 purged allocated nmalloc ndalloc nrequests small: 498088 12262 1272 283989 large: 5414912 11 4 13 total: 5913000 12273 1276 284002 active: 6365184 mapped: 8388608 bins: bin size regs pgs allocated nmalloc ndalloc nrequests nfills nflushes newruns reruns curruns 0 8 501 1 40 109 104 100 3 6 1 0 1 1 16 252 1 164336 10372 101 282735 114 9 41 0 41 2 32 126 1 11232 504 153 751 24 10 4 7 3 3 48 84 1 1200 99 74 181 4 9 1 0 1 4 64 63 1 640 80 70 56 4 6 1 0 1 5 80 50 1 1760 80 58 23 5 6 1 0 1 6 96 84 2 8544 119 30 94 7 7 2 0 2 7 112 72 2 448 80 76 6 3 6 1 0 1 8 128 63 2 768 76 70 19 6 9 1 0 1 [9..10] 11 224 72 4 448 78 76 2 3 6 3 0 1 12 256 63 4 768 72 69 6 4 6 4 0 1 [13..14] 15 448 63 7 28224 63 0 0 1 0 1 0 1 16 512 63 8 1536 69 66 4 3 5 3 0 1 17 640 51 8 0 51 51 1 1 3 1 0 0 18 768 47 9 768 47 46 1 1 3 1 0 1 19 896 45 10 40320 45 0 0 1 0 1 0 1 20 1024 63 16 1024 67 66 4 3 6 2 0 1 [21] 22 1536 42 16 0 47 47 2 2 5 2 0 0 23 1792 38 17 68096 38 0 0 1 0 1 0 1 24 2048 65 33 10240 69 64 2 2 4 1 0 1 25 2560 52 33 17920 58 51 2 2 3 1 0 1 [26] 27 3584 39 35 139776 39 0 0 1 0 1 0 1 large: size pages nmalloc ndalloc nrequests curruns [1] 8192 2 5 3 7 2 [1] 16384 4 2 0 2 2 [3] 32768 8 1 0 1 1 [292] 1232896 301 1 0 1 1 [30] 1359872 332 1 1 1 0 [668] 4100096 1001 1 0 1 1 [17] arenas[1]: assigned threads: 1 dss allocation precedence: disabled dirty pages: 9:0 active:dirty, 2 sweeps, 0 madvises, 0 purged allocated nmalloc ndalloc nrequests small: 3200 100 0 0 large: 32768 1 0 1 total: 35968 101 0 1 active: 36864 mapped: 4194304 bins: bin size regs pgs allocated nmalloc ndalloc nrequests nfills nflushes newruns reruns curruns [0..1] 2 32 126 1 3200 100 0 0 1 0 1 0 1 [3..27] large: size pages nmalloc ndalloc nrequests curruns [7] 32768 8 1 0 1 1 [1010] --- End jemalloc statistics ---
执行结果说明：无，待补充
2.3、MEMORY PURGE 使用方式：
该指令通过调用jemalloc的内部命令尽量把redis进程占用但未有效使用内存释放，即常说的内存碎片释放给操作系统（只适用于使用jemalloc作为allocator的实例）；
2.4、MEMORY STATS 使用方式：
在 redis 4.0 之前，只能通过info memory查看redis实例的内存大体使用状况，而无法了解内存的具体使用细节，比如expire的消耗，client output buffer, query buffer等是很难直观显示的。该指令能够展现redis内部内存使用细节；
执行结果示例及对应参数解析：
1) "peak.allocated" 2) (integer ) 2290242368 3) "total.allocated" 4) (integer ) 2290241776 5) "startup.allocated" 6) (integer ) 6315320 7) "clients.normal" 8) (integer ) 150754 9) "aof.buffer" 10) (integer ) 0 11) "lua.caches" 12) (integer ) 0 13) "db.0" 14) 1) "overhead.hashtable.main" 2) (integer ) 680720488 3) "overhead.hashtable.expires" 4) (integer ) 416 15) "overhead.total" 16) (integer ) 687186978 17) "keys.count" 18) (integer ) 13662569 19) "keys.bytes-per-key" 20) (integer ) 167 21) "dataset.bytes" 22) (integer ) 1603054798 23) "dataset.percentage" 24) "70.188545227050781" 25) "peak.percentage" 26) "99.999977111816406" 27) "allocator.allocated" 28) (integer ) 2291632296 29) "allocator.active" 30) (integer ) 2293006336 31) "allocator.resident" 32) (integer ) 2340564992 33) "allocator-fragmentation.ratio" 34) "1.0005995035171509" 35) "allocator-fragmentation.bytes" 36) (integer ) 1374040 37) "allocator-rss.ratio" 38) "1.0207407474517822" 39) "allocator-rss.bytes" 40) (integer ) 47558656 41) "rss-overhead.ratio" 42) "0.99930697679519653" 43) "rss-overhead.bytes" 44) (integer ) -1622016 45) "fragmentation" 46) "1.0212923288345337" 47) "fragmentation.bytes" 48) (integer ) 48763208
2.5、MEMORY USAGE key [SAMPLES count] 使用方式
memory usage k1 memory usage tk samples 10
其他特点：
只计算value对应的内存估计值，key不存在返回nil；
如果key存在过期，不包含Key Expire的内存占用；
对于编码类型为REDIS_ENCODING_HT和REDIS_ENCODING_SKIPLIST，usage子命令采用类似LRU SAMPLES的抽样方式，默认抽样5个元素求平均 X 元数个数 得出实际内存占用，计算结果是近似值，当面可以指定抽样的SAMPLES个数；
