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From "Earwin Burrfoot (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1609) Eliminate synchronization contention on initial index reading in TermInfosReader ensureIndexIsRead
Date Thu, 23 Apr 2009 16:38:30 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1609?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12701986#action_12701986
] 

Earwin Burrfoot commented on LUCENE-1609:
-----------------------------------------

The problem is not with indexState not being volatile. You can unsafely publish objects that
have no internal state, or their state is consistent enough for you under any memory visibility/reordering
effects. See working example of it in LUCENE-1607, Yonik's hash for interning strings.

The problem is that indexState guards indexTerms, indexInfos, indexPointers, which aren't
volatile too and aren't guarded by the lock. It is possible that one thread does load these
fields and then sets indexState = new IndexRead(), but another thread sees only the last write
and dies with NPE.

> Eliminate synchronization contention on initial index reading in TermInfosReader ensureIndexIsRead

> ---------------------------------------------------------------------------------------------------
>
>                 Key: LUCENE-1609
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1609
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>    Affects Versions: 2.9
>         Environment: Solr 
> Tomcat 5.5
> Ubuntu 2.6.20-17-generic
> Intel(R) Pentium(R) 4 CPU 2.80GHz, 2Gb RAM
>            Reporter: Dan Rosher
>         Attachments: LUCENE-1609.patch
>
>
> synchronized method ensureIndexIsRead in TermInfosReader causes contention under heavy
load
> Simple to reproduce: e.g. Under Solr, with all caches turned off, do a simple range search
e.g. id:[0 TO 999999] on even a small index (in my case 28K docs) and under a load/stress
test application, and later, examining the Thread dump (kill -3) , many threads are blocked
on 'waiting for monitor entry' to this method.
> Rather than using Double-Checked Locking which is known to have issues, this implementation
uses a state pattern, where only one thread can move the object from IndexNotRead state to
IndexRead, and in doing so alters the objects behavior, i.e. once the index is loaded, the
index nolonger needs a synchronized method. 
> In my particular test, this uncreased throughput at least 30 times.

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