usage percentage of Mongodb internal cache

I've ran two performance test on a MongoDB server with identical environment/settings AFAIK. I found that the throughput was 10% apart. When I inspected the mongostat logs, I found that Mongodb in the first test with faster throughput used 95% of its cache_size. In the second test, it stayed at 80%. I rebooted the server system between the runs, but didn't change any of the settings of the server or the client. mongodb log reports the exact same setting in both tests: First test: 2019-08-14T00:42:32.588-0400 I STORAGE [initandlisten] wiredtiger_open config: create,cache_size=96122M,cache_overflow=(file_max=0M),session_max=20000,eviction=(threads_min=4,threads_max=4),config_base=false,statistics=(fast),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000),statistics_log=(wait=0),verbose=(recovery_progress), Second test (after reboot): 2019-08-14T01:11:15.722-0400 I STORAGE [initandlisten] wiredtiger_open config: create,cache_size=96122M,cache_overflow=(file_max=0M),session_max=20000,eviction=(threads_min=4,threads_max=4),config_base=false,statistics=(fast),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000),statistics_log=(wait=0),verbose=(recovery_progress), Here is a sample from mongostat of the first test: insert query update delete getmore command dirty used flushes vsize res qrw arw net_in net_out conn time *0 104209 *0 *0 0 9|0 0.0% 94.9% 0 96.7G 94.7G 107|0 128|0 27.0m 40.2m 272 Aug 14 00:51:13.759 *0 102610 *0 *0 0 0|0 0.0% 94.9% 1 96.7G 94.6G 109|0 128|0 26.6m 39.6m 272 Aug 14 00:51:44.092 *0 104724 *0 *0 0 9|0 0.0% 94.9% 0 96.7G 94.6G 87|0 128|0 27.1m 40.4m 272 Aug 14 00:52:13.767 And a sample from the second test: insert query update delete getmore command dirty used flushes vsize res qrw arw net_in net_out conn time *0 94009 *0 *0 0 9|0 0.0% 80.9% 1 83.1G 80.9G 127|0 81|0 24.4m 36.3m 272 Aug 14 01:17:29.289 *0 93680 *0 *0 0 0|0 0.0% 80.3% 0 83.1G 81.0G 0|0 107|0 24.3m 36.2m 272 Aug 14 01:17:59.119 *0 93068 *0 *0 0 9|0 0.0% 80.9% 1 83.1G 81.0G 130|0 128|0 24.1m 35.9m 272 Aug 14 01:18:29.123 *0 92599 *0 *0 0 0|0 0.0% 80.0% 0 83.1G 80.9G 137|0 128|0 24.0m 35.8m 272 Aug 14 01:18:59.186 - What could have caused the difference in the cache percentage usage and throughput? - is it safe to change explicitly this settings so Mongodb uses more of its cache? - Can I do this setting change from the command line ? (I'm starting mongodb from command line and not as a service)