How to Resolve : "The request took too long to iterate over doc values"?

[raj.yadav]

In our index, we have few fields defined as `ExternalFileField` field type.
We decided to use docValues for such fields. Here is the field type
definition

OLD => (ExternalFileField)
<fieldType name="ext_file_field" indexed="false" stored="true" keyField="id"
defVal="0.0" class="solr.ExternalFileField"/>

NEW => (docValues)
<fieldType name="doc_value_field"  class="solr.FloatPointField"
indexed="false" stored="false"  docValues="true"
useDocValuesAsStored="false"/>

After this modification we started getting the following `timeout warning`
messages:

```The request took too long to iterate over doc values. Timeout: timeoutAt:
1626463774823735 (System.nanoTime(): 1626463774836490),&#8203;
DocValues=org.apache.lucene.codecs.lucene80.Lucene80DocValuesProducer$8@4efddff
```

Our system configuration:
Each Solr Instance: 8 vcpus, 64 GiB memory
JAVA Memory: 30GB
Collection: 4 shards (each shard has approximately 12 million docs and index
size of 12 GB) and each Solr instance has one replica of the shard. 

GC_TUNE="-XX:NewRatio=3 \
-XX:SurvivorRatio=4 \
-XX:PermSize=64m \
-XX:MaxPermSize=64m \
-XX:TargetSurvivorRatio=80 \
-XX:MaxTenuringThreshold=9 \
-XX:+UseConcMarkSweepGC \
-XX:+UseParNewGC \
-XX:+CMSClassUnloadingEnabled \
-XX:ConcGCThreads=4 -XX:ParallelGCThreads=4 \
-XX:+CMSScavengeBeforeRemark \
-XX:PretenureSizeThreshold=64m \
-XX:+UseCMSInitiatingOccupancyOnly \
-XX:CMSInitiatingOccupancyFraction=50 \
-XX:CMSMaxAbortablePrecleanTime=6000 \
-XX:+CMSParallelRemarkEnabled \
-XX:+ParallelRefProcEnabled"
 
1. What this warning message means?
2. How to resolve it?




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