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Date: Wed, 25 Jul 2012 21:16:34 +0000 (UTC)
From: "Thomas Neidhart (JIRA)" <jira@apache.org>
To: issues@commons.apache.org
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Subject: [jira] [Resolved] (COLLECTIONS-425) performance problem in
 ListOrderedMap.remove()
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     [ https://issues.apache.org/jira/browse/COLLECTIONS-425?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Thomas Neidhart resolved COLLECTIONS-425.
-----------------------------------------

       Resolution: Fixed
    Fix Version/s: 4.0

Fixed in r1365749. Thanks for the report and patch!
                
> performance problem in ListOrderedMap.remove()
> ----------------------------------------------
>
>                 Key: COLLECTIONS-425
>                 URL: https://issues.apache.org/jira/browse/COLLECTIONS-425
>             Project: Commons Collections
>          Issue Type: Bug
>    Affects Versions: 3.2.1
>         Environment: java 1.6.0_24
> Ubuntu 11.10
>            Reporter: Adrian Nistor
>             Fix For: 4.0
>
>         Attachments: Test.java, TestWorstCase.java, patch.diff
>
>
> Hi,
> I am encountering a performance problem in ListOrderedMap.remove().
> It appears in version 3.2.1 and also in revision 1365132.  I attached
> a test that exposes this problem and a simple patch that fixes it.  On
> my machine, for this test, the patch provides a 213X speedup.
> To run the test, just do:
> $ java Test
> The output for the un-patched version is:
> Time is 1497
> The output for the patched version is:
> Time is 7
> The patch changes the "ListOrderedMap.remove(Object key)" code from:
> {code:java|borderStyle=solid}
> Object result = getMap().remove(key);
> insertOrder.remove(key);
> {code}
> to:
> {code:java|borderStyle=solid}
> if (decorated().containsKey(key)) {
>     result = decorated().remove(key);
>     insertOrder.remove(key);
> }
> {code}
> If "decorated()" does not contain the key, there is no need to remove
> it.  This change significantly speeds up the code by avoiding the call
> to "insertOrder.remove(key)", which is very expensive because
> "insertOrder" is an ArrayList, and removing from an ArrayList is a
> linear time operation.
> It may appear that the check "if (decorated().containsKey(key))" may
> slow down the code when "decorated()" contains the key, because it
> adds a new operation "decorated().containsKey(key)", without avoiding
> the calls to "getMap().remove(key)" and "insertOrder.remove(key)".
> I attached a test, TestWorstCase.java, that show that, even when
> removing only existing keys (i.e., "decorated().containsKey(key)"
> always returns "true"), the patched version takes almost the same time
> as the un-patched version.
> To run TestWorstCase, just do:
> $ java TestWorstCase
> The output for the un-patched version for TestWorstCase is:
> Time is 96
> The output for the patched version for TestWorstCase is:
> Time is 97
> The reason why the patch does not slow down the code, even for this
> worst case, is because "decorated().containsKey(key)" is a
> "containsKey()" on a HashMap (very fast, constant time operation),
> whereas "insertOrder.remove(key);" is a "remove()" on an ArrayList
> (very slow, linear time operation).  So the time taken by
> "decorated().containsKey(key)" is negligible compared to the time
> taken by "insertOrder.remove(key);".  In other words, most of the time
> is spent inside "insertOrder.remove(key)", and performing one
> additional fast operation cannot be noticed.
> Is this truly a bug? If so, can you please confirm if the patch is
> correct?
> Thanks,
> Adrian

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