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From "Shai Erera (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-3761) Generalize SearcherManager
Date Sat, 11 Feb 2012 05:16:59 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-3761?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13206022#comment-13206022
] 

Shai Erera commented on LUCENE-3761:
------------------------------------

bq. should either be volatile otherwise threads could spin unnecessarily long since tryInc
will fail until the reference is re-read from main memory.

I don't think that we need to make 'current' volatile. It's only changed from swapSearcher
which is synchronized, and therefore as soon as it changes, all shared copies of that instance
(in all threads) gets updated.

There are many web pages that discuss volatile vs. synchronized (just Google those 3 words)
and this page (http://www.javamex.com/tutorials/synchronization_volatile.shtml) even suggests
that volatile is more expensive, because synchronization happens on each access to the variable,
while in synchronized it's only inside the synced block.

We check 'current' in ensureOpen() which happens on every API call, and I think that volatile
would therefore be more expensive. Also, I'm not sure that using AtomicRef would be simpler
code. And following Mike's comments, I prefer to have an explicit synced swapSearcher.
                
> Generalize SearcherManager
> --------------------------
>
>                 Key: LUCENE-3761
>                 URL: https://issues.apache.org/jira/browse/LUCENE-3761
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: core/search
>            Reporter: Shai Erera
>            Assignee: Shai Erera
>            Priority: Minor
>             Fix For: 3.6, 4.0
>
>         Attachments: LUCENE-3761.patch, LUCENE-3761.patch
>
>
> I'd like to generalize SearcherManager to a class which can manage instances of a certain
type of interfaces. The reason is that today SearcherManager knows how to handle IndexSearcher
instances. I have a SearcherManager which manages a pair of IndexSearcher and TaxonomyReader
pair.
> Recently, few concurrency bugs were fixed in SearcherManager, and I realized that I need
to apply them to my version as well. Which led me to think why can't we have an SM version
which is generic enough so that both my version and Lucene's can benefit from?
> The way I see SearcherManager, it can be divided into two parts: (1) the part that manages
the logic of acquire/release/maybeReopen (i.e., ensureOpen, protect from concurrency stuff
etc.), and (2) the part which handles IndexSearcher, or my SearcherTaxoPair. I'm thinking
that if we'll have an interface with incRef/decRef/tryIncRef/maybeRefresh, we can make SearcherManager
a generic class which handles this interface.
> I will post a patch with the initial idea, and we can continue from there.

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