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From "Michael Garski (JIRA)" <>
Subject [jira] Commented: (LUCENENET-190) 2.4.0 Performance in TermInfosReader term caching (New implementation of SimpleLRUCache)
Date Fri, 14 Aug 2009 23:05:15 GMT


Michael Garski commented on LUCENENET-190:


I only performed the test once for each variation.  The 18 hour test run was specifically
for looking at the utilization of the Gen2 heap as any cached items will end up there.  From
my perspecitve I see the results of 11.59 - 12.13 searches/sec as being equivalent.  They
may not be in a true sense, but for the purpose of the tests I ran they are very close.

The big O on the SortedList.Remove is what is contained in MSDN :

With our indexes being so large we are going to continue with the cache disabled, however
I do not mean this to be interpreted as disabling the cache in Lucene.Net across the board.
 Modifying the cache to be more efficient would be the way to proceed to maintain parity with
Java Lucene, perhaps providing a way to disable it, but having it enabled by default.  Either
your patch w/o locking or a minor modification to the currently committed code to use the
underlying base.Map.ContainsKey to check for existence in the list would be appropriate as
at a minimum they keep performance on par with 2.3 for a large index.  Performance metrics
with smaller indexes would be interesting to see and hopefully I'll get a chance to get to
that next week.

While the complexity of the operations within C5 may not change based on the number of items
in the list, I do believe there is a fair amount of overhead in them, which would explain
the performance being similar to other tests.  I don't think including C5 with Lucene would
be appropriate, especially given my results.  I included that variation just to see how it
would perform.  We use the C5 collections in our search system for things such as dynamic
pooled filters, and I was curious to see how it would perform in this case.


> 2.4.0 Performance in TermInfosReader term caching (New implementation of SimpleLRUCache)
> ----------------------------------------------------------------------------------------
>                 Key: LUCENENET-190
>                 URL:
>             Project: Lucene.Net
>          Issue Type: Improvement
>         Environment: v2.4.0
>            Reporter: Digy
>            Priority: Minor
>         Attachments: cache_Gen2.PNG, SimpleLRUCache.rar
> 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
> {quote}
> 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
> Michael Garski
> Search Architect
> <>
> {quote}

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