Just to not cause any confusion: The last dip where indexing rate dropped from 500 to 0 was because of a disrupton by me connecting to my server running logstash.
This made me think that, could it possibly be that Logstash is at fault here, and not Elasticsearch?
Resource usage for the last 24h:
Since the decline is constant, I have taken a new excerpt of the gc.log:
[2021-03-29T13:00:28.322+0000][4453][gc,start ] GC(10) Pause Young (Concurrent Start) (G1 Evacuation Pause)
[2021-03-29T13:00:28.322+0000][4453][gc,task ] GC(10) Using 2 workers of 2 for evacuation
[2021-03-29T13:00:28.322+0000][4453][gc,age ] GC(10) Desired survivor size 27262976 bytes, new threshold 15 (max threshold 15)
[2021-03-29T13:00:28.349+0000][4453][gc,age ] GC(10) Age table with threshold 15 (max threshold 15)
[2021-03-29T13:00:28.349+0000][4453][gc,age ] GC(10) - age 1: 18247048 bytes, 18247048 total
[2021-03-29T13:00:28.349+0000][4453][gc,age ] GC(10) - age 2: 8488952 bytes, 26736000 total
[2021-03-29T13:00:28.349+0000][4453][gc,phases ] GC(10) Pre Evacuate Collection Set: 0.2ms
[2021-03-29T13:00:28.349+0000][4453][gc,phases ] GC(10) Merge Heap Roots: 0.1ms
[2021-03-29T13:00:28.349+0000][4453][gc,phases ] GC(10) Evacuate Collection Set: 25.4ms
[2021-03-29T13:00:28.349+0000][4453][gc,phases ] GC(10) Post Evacuate Collection Set: 1.4ms
[2021-03-29T13:00:28.349+0000][4453][gc,phases ] GC(10) Other: 0.2ms
[2021-03-29T13:00:28.349+0000][4453][gc,heap ] GC(10) Eden regions: 99->0(95)
[2021-03-29T13:00:28.349+0000][4453][gc,heap ] GC(10) Survivor regions: 3->7(13)
[2021-03-29T13:00:28.349+0000][4453][gc,heap ] GC(10) Old regions: 14->14
[2021-03-29T13:00:28.349+0000][4453][gc,heap ] GC(10) Archive regions: 2->2
[2021-03-29T13:00:28.349+0000][4453][gc,heap ] GC(10) Humongous regions: 2->2
[2021-03-29T13:00:28.349+0000][4453][gc,metaspace] GC(10) Metaspace: 93718K(98608K)->93718K(98608K) NonClass: 80978K(84580K)->80978K(84580K) Class: 12739K(14028K)->12739K(14028K)
[2021-03-29T13:00:28.349+0000][4453][gc ] GC(10) Pause Young (Concurrent Start) (G1 Evacuation Pause) 474M->93M(8192M) 27.536ms
[2021-03-29T13:00:28.349+0000][4453][gc,cpu ] GC(10) User=0.05s Sys=0.00s Real=0.02s
[2021-03-29T13:00:28.349+0000][4453][safepoint ] Safepoint "G1CollectForAllocation", Time since last: 584343059 ns, Reaching safepoint: 192601 ns, At safepoint: 27598857 ns, Total: 27791458 ns
[2021-03-29T13:00:28.349+0000][4453][gc ] GC(11) Concurrent Cycle
[2021-03-29T13:00:28.349+0000][4453][gc,marking ] GC(11) Concurrent Car Claimed Marks
[2021-03-29T13:00:28.349+0000][4453][gc,marking ] GC(11) Concurrent Clear Claimed Marks 0.072ms
[2021-03-29T13:00:28.349+0000][4453][gc,marking ] GC(11) Concurrent Scan Root Regions
[2021-03-29T13:00:28.376+0000][4453][gc,marking ] GC(11) Concurrent Scan Root Regions 27.019ms
[2021-03-29T13:00:28.376+0000][4453][gc,marking ] GC(11) Concurrent Mark (523.406s)
[2021-03-29T13:00:28.376+0000][4453][gc,marking ] GC(11) Concurrent Mark From Roots
[2021-03-29T13:00:28.376+0000][4453][gc,task ] GC(11) Using 1 workers of 1 for marking
[2021-03-29T13:00:28.494+0000][4453][gc,marking ] GC(11) Concurrent Mark From Roots 117.692ms
[2021-03-29T13:00:28.494+0000][4453][gc,marking ] GC(11) Concurrent Preclean
[2021-03-29T13:00:28.494+0000][4453][gc,marking ] GC(11) Concurrent Preclean 0.095ms
[2021-03-29T13:00:28.494+0000][4453][gc,marking ] GC(11) Concurrent Mark (523.406s, 523.524s) 117.832ms
[2021-03-29T13:00:28.494+0000][4453][gc,start ] GC(11) Pause Remark
[2021-03-29T13:00:28.498+0000][4453][gc ] GC(11) Pause Remark 105M->105M(8192M) 4.160ms
[2021-03-29T13:00:28.498+0000][4453][gc,cpu ] GC(11) User=0.01s Sys=0.00s Real=0.00s
[2021-03-29T13:00:28.499+0000][4453][safepoint ] Safepoint "G1Concurrent", Time since last: 145013651 ns, Reaching safepoint: 112037 ns, At safepoint: 4236346 ns, Total: 4348383 ns
[2021-03-29T13:00:28.499+0000][4453][gc,marking ] GC(11) Concurrent Rebuild Remembered Sets
[2021-03-29T13:00:28.548+0000][4453][gc,marking ] GC(11) Concurrent Rebuild Remembered Sets 49.529ms
[2021-03-29T13:00:28.548+0000][4453][gc,start ] GC(11) Pause Cleanup
[2021-03-29T13:00:28.549+0000][4453][gc ] GC(11) Pause Cleanup 105M->105M(8192M) 0.239ms
[2021-03-29T13:00:28.549+0000][4453][gc,cpu ] GC(11) User=0.00s Sys=0.00s Real=0.00s
[2021-03-29T13:00:28.549+0000][4453][safepoint ] Safepoint "G1Concurrent", Time since last: 49648777 ns, Reaching safepoint: 303783 ns, At safepoint: 280670 ns, Total: 584453 ns
[2021-03-29T13:00:28.549+0000][4453][gc,marking ] GC(11) Concurrent Cleanup for Next Mark
[2021-03-29T13:00:28.558+0000][4453][gc,marking ] GC(11) Concurrent Cleanup for Next Mark 9.200ms
[2021-03-29T13:00:28.558+0000][4453][gc ] GC(11) Concurrent Cycle 208.850ms
[2021-03-29T13:00:29.549+0000][4453][safepoint ] Safepoint "Cleanup", Time since last: 1000117144 ns, Reaching safepoint: 101291 ns, At safepoint: 7687 ns, Total: 108978 ns
[2021-03-29T13:00:30.549+0000][4453][safepoint ] Safepoint "Cleanup", Time since last: 1000128221 ns, Reaching safepoint: 96023 ns, At safepoint: 4341 ns, Total: 100364 ns
[2021-03-29T13:00:31.262+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 712617697 ns, Reaching safepoint: 152530 ns, At safepoint: 3016 ns, Total: 155546 ns
[2021-03-29T13:00:31.557+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 294541868 ns, Reaching safepoint: 235063 ns, At safepoint: 4340 ns, Total: 239403 ns
[2021-03-29T13:00:31.848+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 291074464 ns, Reaching safepoint: 261530 ns, At safepoint: 15002 ns, Total: 276532 ns
[2021-03-29T13:00:32.429+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 580502902 ns, Reaching safepoint: 253730 ns, At safepoint: 13846 ns, Total: 267576 ns
[2021-03-29T13:00:32.622+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 192603819 ns, Reaching safepoint: 206647 ns, At safepoint: 4427 ns, Total: 211074 ns
[2021-03-29T13:00:32.950+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 328093089 ns, Reaching safepoint: 239085 ns, At safepoint: 11953 ns, Total: 251038 ns
[2021-03-29T13:00:33.356+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 405525178 ns, Reaching safepoint: 298447 ns, At safepoint: 10766 ns, Total: 309213 ns
[2021-03-29T13:00:33.706+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 349973687 ns, Reaching safepoint: 129744 ns, At safepoint: 5533 ns, Total: 135277 ns
[2021-03-29T13:00:34.709+0000][4453][safepoint ] Safepoint "Cleanup", Time since last: 1003279168 ns, Reaching safepoint: 145472 ns, At safepoint: 4090 ns, Total: 149562 ns
[2021-03-29T13:00:35.025+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 315357798 ns, Reaching safepoint: 151470 ns, At safepoint: 5516 ns, Total: 156986 ns
[2021-03-29T13:00:35.543+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 517606601 ns, Reaching safepoint: 319444 ns, At safepoint: 16871 ns, Total: 336315 ns
[2021-03-29T13:00:36.352+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 808676406 ns, Reaching safepoint: 152365 ns, At safepoint: 10883 ns, Total: 163248 ns
[2021-03-29T13:00:37.353+0000][4453][safepoint ] Safepoint "Cleanup", Time since last: 1000747661 ns, Reaching safepoint: 133825 ns, At safepoint: 1992 ns, Total: 135817 ns
[2021-03-29T13:00:38.353+0000][4453][safepoint ] Safepoint "Cleanup", Time since last: 1000191379 ns, Reaching safepoint: 232132 ns, At safepoint: 4277 ns, Total: 236409 ns
[2021-03-29T13:00:39.604+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 1251336447 ns, Reaching safepoint: 161366 ns, At safepoint: 13150 ns, Total: 174516 ns
[2021-03-29T13:00:39.910+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 304846125 ns, Reaching safepoint: 249663 ns, At safepoint: 7954 ns, Total: 257617 ns
[2021-03-29T13:00:40.612+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 702128749 ns, Reaching safepoint: 266270 ns, At safepoint: 12526 ns, Total: 278796 ns
[2021-03-29T13:00:41.032+0000][4453][gc,start ] GC(12) Pause Young (Normal) (G1 Evacuation Pause)
[2021-03-29T13:00:41.032+0000][4453][gc,task ] GC(12) Using 2 workers of 2 for evacuation
[2021-03-29T13:00:41.032+0000][4453][gc,age ] GC(12) Desired survivor size 27262976 bytes, new threshold 15 (max threshold 15)
[2021-03-29T13:00:41.067+0000][4453][gc,age ] GC(12) Age table with threshold 15 (max threshold 15)
[2021-03-29T13:00:41.067+0000][4453][gc,age ] GC(12) - age 1: 18040896 bytes, 18040896 total
[2021-03-29T13:00:41.067+0000][4453][gc,age ] GC(12) - age 2: 17721776 bytes, 35762672 total
[2021-03-29T13:00:41.067+0000][4453][gc,age ] GC(12) - age 3: 8474520 bytes, 44237192 total
[2021-03-29T13:00:41.067+0000][4453][gc,phases ] GC(12) Pre Evacuate Collection Set: 1.8ms
[2021-03-29T13:00:41.067+0000][4453][gc,phases ] GC(12) Merge Heap Roots: 0.2ms
[2021-03-29T13:00:41.067+0000][4453][gc,phases ] GC(12) Evacuate Collection Set: 30.8ms
[2021-03-29T13:00:41.067+0000][4453][gc,phases ] GC(12) Post Evacuate Collection Set: 2.4ms
[2021-03-29T13:00:41.067+0000][4453][gc,phases ] GC(12) Other: 0.2ms
[2021-03-29T13:00:41.068+0000][4453][gc,heap ] GC(12) Eden regions: 95->0(91)
[2021-03-29T13:00:41.068+0000][4453][gc,heap ] GC(12) Survivor regions: 7->11(13)
[2021-03-29T13:00:41.068+0000][4453][gc,heap ] GC(12) Old regions: 14->14
[2021-03-29T13:00:41.068+0000][4453][gc,heap ] GC(12) Archive regions: 2->2
[2021-03-29T13:00:41.068+0000][4453][gc,heap ] GC(12) Humongous regions: 2->2
[2021-03-29T13:00:41.068+0000][4453][gc,metaspace] GC(12) Metaspace: 100146K(105008K)->100146K(105008K) NonClass: 86856K(90468K)->86856K(90468K) Class: 13289K(14540K)->13289K(14540K)
[2021-03-29T13:00:41.068+0000][4453][gc ] GC(12) Pause Young (Normal) (G1 Evacuation Pause) 473M->110M(8192M) 35.430ms
[2021-03-29T13:00:41.068+0000][4453][gc,cpu ] GC(12) User=0.05s Sys=0.01s Real=0.03s
[2021-03-29T13:00:41.068+0000][4453][safepoint ] Safepoint "G1CollectForAllocation", Time since last: 413497131 ns, Reaching safepoint: 6580750 ns, At safepoint: 35500491 ns, Total: 42081241 ns
[2021-03-29T13:00:41.210+0000][4453][safepoint ] Safepoint "ICBufferFull", Time since last: 142169687 ns, Reaching safepoint: 241097 ns, At safepoint: 6226 ns, Total: 247323 ns