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https://issues.apache.org/jira/browse/SOLR-665?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12617509#action_12617509
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Mark Miller commented on SOLR-665:
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Sent to the mailing List:
>>Depends on if you want to go by the javadoc or not - it says that if *any* of
>>the threads accessing the map concurrently make a structural change, the map
>>must be synchronized. It's clear that get does not >>make a structural
>>change when using insertion order, but can any other threads possibly make
>>a mapping change while calling get? Sounds like yes, and so the contract
>>would seem to indicate you >>synchronize...
>>Put can modify shared variables that get accesses - sounds like dangerous
>>ground to me. If it works, its got to be sneaky enough to warrant code
>>smell...
>>- Mark
Further:
Here is just one of possibly many problems - a put call can resize and make an
entirely new table array. A get call can do something like the following with
the table array:
table[indexFor(hash, table.length)
Do to execution reordering / memory barriers, would seem to me that the table
being indexed into and the table.length may not be the values you were hoping
for...
> FIFO Cache (Unsynchronized): 9x times performance boost
> -------------------------------------------------------
>
> Key: SOLR-665
> URL: https://issues.apache.org/jira/browse/SOLR-665
> Project: Solr
> Issue Type: Improvement
> Affects Versions: 1.3
> Environment: JRockit R27 (Java 6)
> Reporter: Fuad Efendi
> Attachments: FIFOCache.java
>
> Original Estimate: 672h
> Remaining Estimate: 672h
>
> Attached is modified version of LRUCache where
> 1. map = new LinkedHashMap(initialSize, 0.75f, false) - so that
> "reordering"/true (performance bottleneck of LRU) is replaced to
> "insertion-order"/false (so that it became FIFO)
> 2. Almost all (absolutely unneccessary) synchronized statements commented out
> See discussion at
> http://www.nabble.com/LRUCache---synchronized%21--td16439831.html
> Performance metrics (taken from SOLR Admin):
> LRU
> Requests: 7638
> Average Time-Per-Request: 15300
> Average Request-per-Second: 0.06
> FIFO:
> Requests: 3355
> Average Time-Per-Request: 1610
> Average Request-per-Second: 0.11
> Performance increased 9 times which roughly corresponds to a number of CPU in
> a system, http://www.tokenizer.org/ (Shopping Search Engine at Tokenizer.org)
> Current number of documents: 7494689
> name: filterCache
> class: org.apache.solr.search.LRUCache
> version: 1.0
> description: LRU Cache(maxSize=10000000, initialSize=1000)
> stats: lookups : 15966954582
> hits : 16391851546
> hitratio : 0.102
> inserts : 4246120
> evictions : 0
> size : 2668705
> cumulative_lookups : 16415839763
> cumulative_hits : 16411608101
> cumulative_hitratio : 0.99
> cumulative_inserts : 4246246
> cumulative_evictions : 0
> Thanks
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