从3.2开始，monogdb的默认存储引擎就是WiredTiger。mongodb会同时使用wt的内部缓存和文件系统缓存。从3.4开始，mongodb的wt引擎内存缓存的设置规则为：(RAM-1GB)*50% 或者256MB
你需要分析是否对默认的内存做调优。一条比较好的原则就是wt的缓存足够大，能够缓存整个应用的工作集。
查看wt的缓存统计信息：
db.serverStatus().wiredTiger.cache
{
"application threads page read from disk to cache count" : 9,
"application threads page read from disk to cache time (usecs)" : 17555,
"application threads page write from cache to disk count" : 1820,
"application threads page write from cache to disk time (usecs)" : 1052322,
"bytes allocated for updates" : 20043,
"bytes belonging to page images in the cache" : 46742,
"bytes belonging to the history store table in the cache" : 173,
"bytes currently in the cache" : 73044,
"bytes dirty in the cache cumulative" : 38638327,
"bytes not belonging to page images in the cache" : 26302,
"bytes read into cache" : 43280,
"bytes written from cache" : 20517382,
"cache overflow score" : 0,
"checkpoint blocked page eviction" : 0,
"eviction calls to get a page" : 5973,
"eviction calls to get a page found queue empty" : 4973,
"eviction calls to get a page found queue empty after locking" : 20,
"eviction currently operating in aggressive mode" : 0,
"eviction empty score" : 0,
"eviction passes of a file" : 0,
"eviction server candidate queue empty when topping up" : 0,
"eviction server candidate queue not empty when topping up" : 0,
"eviction server evicting pages" : 0,
"eviction server slept, because we did not make progress with eviction" : 735,
"eviction server unable to reach eviction goal" : 0,
"eviction server waiting for a leaf page" : 2,
"eviction state" : 64,
"eviction walk target pages histogram - 0-9" : 0,
"eviction walk target pages histogram - 10-31" : 0,
"eviction walk target pages histogram - 128 and higher" : 0,
"eviction walk target pages histogram - 32-63" : 0,
"eviction walk target pages histogram - 64-128" : 0,
"eviction walk target strategy both clean and dirty pages" : 0,
"eviction walk target strategy only clean pages" : 0,
"eviction walk target strategy only dirty pages" : 0,
"eviction walks abandoned" : 0,
"eviction walks gave up because they restarted their walk twice" : 0,
"eviction walks gave up because they saw too many pages and found no candidates" : 0,
"eviction walks gave up because they saw too many pages and found too few candidates" : 0,
"eviction walks reached end of tree" : 0,
"eviction walks started from root of tree" : 0,
"eviction walks started from saved location in tree" : 0,
"eviction worker thread active" : 4,
"eviction worker thread created" : 0,
"eviction worker thread evicting pages" : 902,
"eviction worker thread removed" : 0,
"eviction worker thread stable number" : 0,
"files with active eviction walks" : 0,
"files with new eviction walks started" : 0,
"force re-tuning of eviction workers once in a while" : 0,
"forced eviction - history store pages failed to evict while session has history store cursor open" : 0,
"forced eviction - history store pages selected while session has history store cursor open" : 0,
"forced eviction - history store pages successfully evicted while session has history store cursor open" : 0,
"forced eviction - pages evicted that were clean count" : 0,
"forced eviction - pages evicted that were clean time (usecs)" : 0,
"forced eviction - pages evicted that were dirty count" : 0,
"forced eviction - pages evicted that were dirty time (usecs)" : 0,
"forced eviction - pages selected because of too many deleted items count" : 0,
"forced eviction - pages selected count" : 0,
"forced eviction - pages selected unable to be evicted count" : 0,
"forced eviction - pages selected unable to be evicted time" : 0,
"forced eviction - session returned rollback error while force evicting due to being oldest" : 0,
"hazard pointer blocked page eviction" : 0,
"hazard pointer check calls" : 902,
"hazard pointer check entries walked" : 25,
"hazard pointer maximum array length" : 1,
"history store key truncation calls that returned restart" : 0,
"history store key truncation due to mixed timestamps" : 0,
"history store key truncation due to the key being removed from the data page" : 0,
"history store score" : 0,
"history store table insert calls" : 0,
"history store table insert calls that returned restart" : 0,
"history store table max on-disk size" : 0,
"history store table on-disk size" : 0,
"history store table out-of-order resolved updates that lose their durable timestamp" : 0,
"history store table out-of-order updates that were fixed up by moving existing records" : 0,
"history store table out-of-order updates that were fixed up during insertion" : 0,
"history store table reads" : 0,
"history store table reads missed" : 0,
"history store table reads requiring squashed modifies" : 0,
"history store table remove calls due to key truncation" : 0,
"history store table writes requiring squashed modifies" : 0,
"in-memory page passed criteria to be split" : 0,
"in-memory page splits" : 0,
"internal pages evicted" : 0,
"internal pages queued for eviction" : 0,
"internal pages seen by eviction walk" : 0,
"internal pages seen by eviction walk that are already queued" : 0,
"internal pages split during eviction" : 0,
"leaf pages split during eviction" : 0,
"maximum bytes configured" : 8053063680,
"maximum page size at eviction" : 376,
"modified pages evicted" : 902,
"modified pages evicted by application threads" : 0,
"operations timed out waiting for space in cache" : 0,
"overflow pages read into cache" : 0,
"page split during eviction deepened the tree" : 0,
"page written requiring history store records" : 0,
"pages currently held in the cache" : 24,
"pages evicted by application threads" : 0,
"pages queued for eviction" : 0,
"pages queued for eviction post lru sorting" : 0,
"pages queued for urgent eviction" : 902,
"pages queued for urgent eviction during walk" : 0,
"pages read into cache" : 20,
"pages read into cache after truncate" : 902,
"pages read into cache after truncate in prepare state" : 0,
"pages requested from the cache" : 33134,
"pages seen by eviction walk" : 0,
"pages seen by eviction walk that are already queued" : 0,
"pages selected for eviction unable to be evicted" : 0,
"pages selected for eviction unable to be evicted as the parent page has overflow items" : 0,
"pages selected for eviction unable to be evicted because of active children on an internal page" : 0,
"pages selected for eviction unable to be evicted because of failure in reconciliation" : 0,
"pages walked for eviction" : 0,
"pages written from cache" : 1822,
"pages written requiring in-memory restoration" : 0,
"percentage overhead" : 8,
"tracked bytes belonging to internal pages in the cache" : 5136,
"tracked bytes belonging to leaf pages in the cache" : 67908,
"tracked dirty bytes in the cache" : 493,
"tracked dirty pages in the cache" : 1,
"unmodified pages evicted" : 0
}
这里关于WiredTiger缓存的统计有很多，但是我们应该重点关注以下内容：
1.wiredTiger.cache.maximum bytes configured
当前缓存的最大值
2.wiredTiger.cache.bytes currently in the cache
当前在缓存中的数据。通常这个值是你配置的wt的缓存的80%，在加上一些尚未写入磁盘的脏数据。这个统计信息的值不能超过分配给wt的最大值。如果该值接近或超过分配给wt的最大值，则表示应该横向扩展了(scaled out)
3.wiredTiger.cache.tracked dirty bytes in the cache
这个值表示的是wt的缓存中脏数据的大小。应该小于你配置的wt的缓存的5%。也可以作为是否需要进行横向扩展的一个指标。一旦超过5%，就可能会移除更多的缓存数据，某些场景下为了让客户端成功写入数据，会驱除掉缓存数据。
4.wiredTiger.cache.pages read into cache
读入缓存的页数，可以使用该指标来判断每秒读入缓存的数据量。
如果数值较大，表明是读负载比较高的应用。如果一直比较大，那增加内存的大小，可能会增加你的系统的性能。
5.wiredTiger.cache.pages written from cache
从缓存写入磁盘的数据页数。发生检查点的时候，该值会比较大。如果该值继续增大，你的检查点就会变长。
分析这几个统计数据，我们就可以知道是否需要增加内存。
此外，我们也该查看WiredTiger的并发写ticket和读ticket的使用情况。如果数量继续增加，接近cores的数量，那么你的cpu将达到饱和。
可以使用pmm来检查并发写ticket和读ticket的使用情况。
也可以使用以下命令：
db.serverStatus().wiredTiger.concurrentTransactions
{
"write" : {
"out" : 0,
"available" : 128,
"totalTickets" : 128
},
"read" : {
"out" : 1,
"available" : 127,
"totalTickets" : 128
}
}
在我们这个例子中：
·wiredTiger.cache.maximum bytes configured = 8053063680，即7.5 GB
·wiredTiger.cache.bytes currently in the cache = 8053063680，即7.5 GB
·wiredTiger.cache.tracked dirty bytes in the cache = 4294967296，即4 GB
·wiredTiger.cache.pages read into cache = ~ 512 MB
·wiredTiger.cache.pages written from cache = ~ 1 GB
我们使用的是默认配置，所以可以知道我们的内存是16GB。基于上面的分析，我们知道
我们使用默认的wiredTiger缓存大小，因此我们知道系统中有16gb内存(0.5 * (16-1))= 7.5 GB。根据我们对(假想的)应用的了解，我们知道工作集是16gb，所以我们想要比这个数字更大。由于我们的工作集只会继续增长，为了具备额外的增长空间，我们可以将服务器的RAM从16gb调整到48gb。如果我们仍然使用默认设置，这将增加我们的WiredTiger缓存到23.5 GB=(0.5 * (48-1)) 。这将为操作系统及其文件系统缓存留下24.5 GB的RAM。
storage:
wiredTiger:
engineConfig:
cacheSizeGB: 30
对于默认设置或特定设置，要能识别增加的内存并生效，我们将需要重新启动mongod进程。