1.问题出现
某天我发现云服务器的内存使用达到了76%，我的乖乖这了得，我是8G的配置，上面没跑几个服务，这就很不正常。我立马用命令查看了下。
好家伙，原来是mongodb‘独占鳌头’，占用了百分之四十多的内存。
于是我想着法儿来释放内存。
1.1 重启服务可以释放掉，这种方式估计没人会采用。
1.2 使用mongo内置命令也可以达到
- mongo> use admin
- mongo> db.runCommand({closeAllDatabases:1})
1.3 也可以直接调整
- shell> sysctl -w vm.drop_caches=1
等等。。
这些都是当时可以达到效果，但是过了一天，我又发现问题回到了原点，随着使用时间的增加，物理内存会逐步占用升高，治标不治本。
2.发现原因
于是我开始大量查阅在线资料。
有几篇博文讲的蛮全面(但是我又发现大量雷同文章出现)??
这篇应该是原创，时间蛮早的。
最后我确定了我这种小白错误原因：
官网上写的很清楚：https://docs.mongodb.com/v3.4/core/wiredtiger/index.html
- With WiredTiger, MongoDB utilizes both the WiredTiger internal cache and the filesystem cache.
- Starting in 3.4, the WiredTiger internal cache, by default, will use the larger of either:
- 50% of (RAM - 1 GB), or
- 256 MB.
- By default, WiredTiger uses Snappy block compression for all collections and prefix compression for all indexes. Compression defaults are configurable at a global level and can also be set on a per-collection and per-index basis during collection and index creation.
- Different representations are used for data in the WiredTiger internal cache versus the on-disk format:
- Data in the filesystem cache is the same as the on-disk format, including benefits of any compression for data files. The filesystem cache is used by the operating system to reduce disk I/O.
- Indexes loaded in the WiredTiger internal cache have a different data representation to the on-disk format, but can still take advantage of index prefix compression to reduce RAM usage. Index prefix compression deduplicates common prefixes from indexed fields.
- Collection data in the WiredTiger internal cache is uncompressed and uses a different representation from the on-disk format. Block compression can provide significant on-disk storage savings, but data must be uncompressed to be manipulated by the server.
- Via the filesystem cache, MongoDB automatically uses all free memory that is not used by the WiredTiger cache or by other processes.
- To adjust the size of the WiredTiger internal cache, seestorage.wiredTiger.engineConfig.cacheSizeGB and --wiredTigerCacheSizeGB. Avoid increasing the WiredTiger internal cache size above its default value
3.解决方式
原来是忽略这个原因，默认会使用50%的内存。
根据官方说的改改配置文件吧，加上这一段。
mongod.conf
- storage:
wiredTiger: - engineConfig:
- cacheSizeGB: 2.5
2.5 代表2.5G，这这个看你的配置了和需求了。
持续观察后，发现它确实稳定到了我们的所设置的值。这就解决了，爽歪歪。
21:59:52
2021-03-22
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