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From "Digy (JIRA)" <>
Subject [jira] Created: (LUCENENET-190) 2.4.0 Performance in TermInfosReader term caching (New implementation of SimpleLRUCache)
Date Mon, 10 Aug 2009 12:02:14 GMT
2.4.0 Performance in TermInfosReader term caching (New implementation of SimpleLRUCache)

                 Key: LUCENENET-190
             Project: Lucene.Net
          Issue Type: Improvement
         Environment: v2.4.0
            Reporter: Digy
            Priority: Minor

Below is the mail from Michael Garski about the Performance in TermInfosReader term caching.
It would be good to have a faster LRUCache implementation in Lucene.Net


Doug did an amazing job of porting 2.4.0, doing it mostly on his own!  
Hooray Doug!

We are using the committed version of 2.4.0 in production and I wanted to share a performance
issue we discovered and what we've done to work around it.  From the Java Lucene change log:
 "LUCENE-1195: Improve term lookup performance by adding a LRU cache to the TermInfosReader.
In performance experiments the speedup was about 25% on average on mid-size indexes with ~500,000
documents for queries with 3 terms and about 7% on larger indexes with ~4.3M documents."

The Java implementation uses a LinkedHashMap within the class org.apache.lucene.util.cache.SimpleLRUCache,
which is very efficient at maintaining the cache.  As there is no equivalent collection in
.Net The current 2.4.0 port uses a combination of a LinkedList to maintain LRU state and a
HashTable to provide lookups.  While this implementation works, maintaining the LRU state
via the LinkedList creates a fair amount of overhead and can result in a significant reduction
of performance, most likely attributed to the LinkedList.Remove method being O(n).  As each
thread maintains its own cache of 1024 terms, these overhead in performing the removal is
a drain on performance.

At this time we have disabled the cache in the method TermInfosReader.TermInfo Get(Term term,
bool useCache) by always setting the useCache parameter to false inside the body of the method.
 After doing this we saw performance return back to the 2.3.2 levels.  I have not yet had
the opportunity to experiment with other implementations within the SimpleLRUCache to address
the performance issue.  One approach that would might solve the issue is to use the HashedLinkedList<T>
class provided in the C5 collection library [].


Michael Garski
Search Architect <>


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