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FAQ - Solr logging "PERFORMANCE WARNING: Overlapping onDeckSearchers" and its meaning

Summary

This article discusses what the warning about overlapping onDeckSearchers means and how to remedy it

Symptoms

The user may see the following entry in their log files which is often accompanied by performance degradation and potentially higher heap memory usage:

WARN  [commitScheduler-4-thread-1] 2015-12-16 08:46:42,488  SolrCore.java:1712 - [mykeyspace.my_search_table] PERFORMANCE WARNING: Overlapping onDeckSearchers=2

Cause

When a commit is issued to a Solr core, it makes index changes visible to new search requests. Commits may come from an application or an auto-commit. A "normal" commit in DSE is usually more often than not from an auto commit in which is, as outlined here, configured in the solr config file.

Each time a commit is issued a new searcher object is created. When there are too many searcher objects this warning will be observed.

Also if the configuration is such that a searcher has pre-warming queries, this can delay the start time meaning that the searcher is still starting up when a new commit comes in.

Solution

Decrease overlapping commits

Find out if there are commits being issued from the application. These might overlap with the auto commits. Ideally one would tune the auto commit settings to suit the application, negating the need for anything but auto commits.

Reduce auto warm count (if not using SolrFilterCache)

If you are not using the default SolrFilterCache Disable or reduce the autowarmCount setting for the given filter cache you are using. This setting controls the amount of objects populated from an older cache.

Increase the max searchers

One can increase the following setting in the solr_config.xml if required

<maxWarmingSearchers>16</maxWarmingSearchers>

Note: Having too high a number here can place more load on the node and have a negative impact on performance. It is normally recommended to keep the at 50% number of CPUs at the most. In most cases the default of 2 should be sufficient anyway. 

Further reading

The subject is also covered here in the Solr FAQ: 

FAQ What_does exceeded_limit_of_maxWarmingSearchers mean?

Solr wiki - autowarmCount

 

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