change default number of segment loading threads

docs/configuration/index.md

@@ -1383,7 +1383,7 @@ These Historical configurations can be defined in the `historical/runtime.proper
 |`druid.segmentCache.dropSegmentDelayMillis`|How long a process delays before completely dropping segment.|30000 (30 seconds)|
 |`druid.segmentCache.infoDir`|Historical processes keep track of the segments they are serving so that when the process is restarted they can reload the same segments without waiting for the Coordinator to reassign. This path defines where this metadata is kept. Directory will be created if needed.|${first_location}/info_dir|
 |`druid.segmentCache.announceIntervalMillis`|How frequently to announce segments while segments are loading from cache. Set this value to zero to wait for all segments to be loaded before announcing.|5000 (5 seconds)|
-|`druid.segmentCache.numLoadingThreads`|How many segments to drop or load concurrently from deep storage. Note that the work of loading segments involves downloading segments from deep storage, decompressing them and loading them to a memory mapped location. So the work is not all I/O Bound. Depending on CPU and network load, one could possibly increase this config to a higher value.|Number of cores|
+|`druid.segmentCache.numLoadingThreads`|How many segments to drop or load concurrently from deep storage. Note that the work of loading segments involves downloading segments from deep storage, decompressing them and loading them to a memory mapped location. So the work is not all I/O Bound. Depending on CPU and network load, one could possibly increase this config to a higher value.|max(1,Number of cores / 6)|
 |`druid.segmentCache.numBootstrapThreads`|How many segments to load concurrently during historical startup.|`druid.segmentCache.numLoadingThreads`|
 |`druid.segmentCache.lazyLoadOnStart`|Whether or not to load segment columns metadata lazily during historical startup. When set to true, Historical startup time will be dramatically improved by deferring segment loading until the first time that segment takes part in a query, which will incur this cost instead. One catch is that if historical crashes while in the process of downloading and creating segment files, it is possible to end up with a corrupted segment on disk, this requires manual intervention to delete corrupted files. When the flag is set to true, historical startup would complete successfully and queries using this segment would fail at runtime.|false|
 |`druid.coordinator.loadqueuepeon.curator.numCallbackThreads`|Number of threads for executing callback actions associated with loading or dropping of segments. One might want to increase this number when noticing clusters are lagging behind w.r.t. balancing segments across historical nodes.|2|

server/src/main/java/org/apache/druid/segment/loading/SegmentLoaderConfig.java

@@ -50,7 +50,7 @@ public class SegmentLoaderConfig
   private int announceIntervalMillis = 0; // do not background announce
 
   @JsonProperty("numLoadingThreads")
-  private int numLoadingThreads = JvmUtils.getRuntimeInfo().getAvailableProcessors();
+  private int numLoadingThreads = Math.max(1, JvmUtils.getRuntimeInfo().getAvailableProcessors() / 6);
 
   @JsonProperty("numBootstrapThreads")
   private Integer numBootstrapThreads = null;