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https://issues.apache.org/jira/browse/LUCENE-3653?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13172258#comment-13172258
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Uwe Schindler commented on LUCENE-3653:
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The code for the commit was standing here more than 24 hours and was tested
extensively to remove contention on creation on all classes using VirtualMethod
(IndexSearcher and TokenStream are only example). What's your problem with it?
The latest commit 1 hr ago was only a fix in the tests and had nothing to do
with ConcurrentHashMap.
As I said, CHM is used for read-access only, but the guard is needed, if two
ctors are running at same time and need to add a new entry to cache. All other
cases are read-only, so lockless is better. It is just stupid to wait on a
ctor, just because there is a lock on a map thats never be updated once your
code is running an no new classes using VirtualMethod are loaded. We can also
use Google Guava to use their MapMaker or wait until Java 8, where Doug Lea's
ConcurrentWeakHashMap is maybe added (JSR-166).
In fact the code of AttributeSource got simplier because we have no sync blocks.
> Lucene Search not scalling
> --------------------------
>
> Key: LUCENE-3653
> URL: https://issues.apache.org/jira/browse/LUCENE-3653
> Project: Lucene - Java
> Issue Type: Improvement
> Reporter: Gerrit Jansen van Vuuren
> Attachments: App.java,
> LUCENE-3653-VirtualMethod+AttributeSource.patch,
> LUCENE-3653-VirtualMethod+AttributeSource.patch,
> LUCENE-3653-VirtualMethod+AttributeSource.patch, LUCENE-3653-no-sync.png,
> LUCENE-3653-sync-.png, LUCENE-3653.patch,
> LUCENE-3653.patch-BiasedLockingStartupDelay_1.png,
> LUCENE-3653.patch-BiasedLockingStartupDelay_2.png,
> LUCENE-3653.patch-BiasedLockingStartupDelay_3.png,
> Threads-LUCENE-3653.patch.png, lucene-unsync.diff, profile_1_a.png,
> profile_1_b.png, profile_1_c.png, profile_1_d.png, profile_2_a.png,
> profile_2_b.png, profile_2_c.png
>
>
> I've noticed that when doing thousands of searches in a single thread the
> average time is quite low i.e. a few milliseconds. When adding more
> concurrent searches doing exactly the same search the average time increases
> drastically.
> I've profiled the search classes and found that the whole of lucene blocks on
> org.apache.lucene.index.SegmentCoreReaders.getTermsReader
> org.apache.lucene.util.VirtualMethod
> public synchronized int getImplementationDistance
> org.apache.lucene.util.AttributeSourcew.getAttributeInterfaces
> These cause search times to increase from a few milliseconds to up to 2
> seconds when doing 500 concurrent searches on the same in memory index. Note:
> That the index is not being updates at all, so not refresh methods are called
> at any stage.
> Some questions:
> Why do we need synchronization here?
> There must be a non-lockable solution for these, they basically cause
> lucene to be ok for single thread applications but disastrous for any
> concurrent implementation.
> I'll do some experiments by removing the synchronization from the methods of
> these classes.
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