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From "Fleur Kelpin (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (XALANJ-2540) Very inefficient default behaviour for looking up DTMManager
Date Thu, 04 Jul 2013 19:39:48 GMT

    [ https://issues.apache.org/jira/browse/XALANJ-2540?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13700323#comment-13700323
] 

Fleur Kelpin commented on XALANJ-2540:
--------------------------------------

It's worse. The lookup opens a ZipFile and this synchronizes my threads across different one-per-thread
precompiled xpathexpression evals.
                
> Very inefficient default behaviour for looking up DTMManager
> ------------------------------------------------------------
>
>                 Key: XALANJ-2540
>                 URL: https://issues.apache.org/jira/browse/XALANJ-2540
>             Project: XalanJ2
>          Issue Type: Improvement
>      Security Level: No security risk; visible to anyone(Ordinary problems in Xalan projects.
 Anybody can view the issue.) 
>          Components: DTM, XPath
>    Affects Versions: 2.7.1, 2.7
>            Reporter: Lukas Eder
>
> I have analysed an issue that has been bothering me for some time. When executing XPath
evaluations, it looks like a very significant amount of time is spent in the initialisation
of the XPathContext. I have asked this question on Stack Overflow and answered it myself:
> http://stackoverflow.com/questions/6340802/java-xpath-apache-jaxp-implementation-performance
> I think the default behaviour of 
> org.apache.xml.dtm.ObjectFactory.lookUpFactoryClassName() is quite sub-optimal and should
be improved, statically. I imagine, it is unlikely that this configuration is going to change
once classes have been loaded. Hence, the fallback lookup of META-INF/service/org.apache.xml.dtm.DTMManager
should only be done once.
> For reference, here's the question and answer again in JIRA:
> ----
> I have come to an astonishing conclusion that this:
>     Element e = (Element) document.getElementsByTagName("SomeElementName").item(0);
>     String result = ((Element) e).getTextContent();
> Seems to be an incredible 100x faster than this:
>     // Accounts for 30%, can be cached
>     XPathFactory factory = XPathFactory.newInstance();
>     // Negligible
>     XPath xpath = factory.newXPath();
>     // Accounts for 70% (caching a compiled expression doesn't change much...)
>     String result = (String) xpath.evaluate(
>       "//SomeElementName", document, XPathConstants.STRING);
> I'm using the JVM's default implementation of JAXP:
> org.apache.xpath.jaxp.XPathFactoryImpl
> org.apache.xpath.jaxp.XPathImpl
> I'm really confused, because it's easy to see how JAXP could optimise the above XPath
query to actually execute a simple getElementsByTagName() instead. But it doesn't seem to
do that. This problem is limited to around 5-6 frequently used XPath calls, that are abstracted
and hidden by an API. Those queries involve simple paths (e.g. /a/b/c, no variables, conditions)
against an always available DOM Document only. So, if an optimisation can be done, it will
be quite easy to achieve.
> ----
> I have debugged and profiled my test-case and Xalan/JAXP in general. I managed to identify
the big major problem in
> org.apache.xml.dtm.ObjectFactory.lookUpFactoryClassName()
> It can be seen that every one of the 10k test XPath evaluations led to the classloader
trying to lookup the DTMManager instance in some sort of default configuration. This configuration
is not loaded into memory but accessed every time. Furthermore, this access seems to be protected
by a lock on the ObjectFactory.class itself. When the access fails (by default), then the
configuration is loaded from the xalan.jar file's
> META-INF/service/org.apache.xml.dtm.DTMManager
> configuration file. Every time!:
> Fortunately, this behaviour can be overridden by specifying a JVM parameter like this:
> -Dorg.apache.xml.dtm.DTMManager=
>   org.apache.xml.dtm.ref.DTMManagerDefault
> or
> -Dcom.sun.org.apache.xml.internal.dtm.DTMManager=
>   com.sun.org.apache.xml.internal.dtm.ref.DTMManagerDefault
> So here's a performance improvement overview for 10k consecutive XPath evaluations of
//SomeNodeName against a 90k XML file (measured with System.nanoTime():
> measured library        : Xalan 2.7.0 | Xalan 2.7.1 | Saxon-HE 9.3 | jaxen 1.1.3   
> --------------------------------------------------------------------------------
> without optimisation    :     10400ms |      4717ms |              |     25500ms
> reusing XPathFactory    :      5995ms |      2829ms |              |
> reusing XPath           :      5900ms |      2890ms |              |
> reusing XPathExpression :      5800ms |      2915ms |      16000ms |     25000ms
> adding the JVM param    :      1163ms |       761ms |        n/a   |

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