plugins/org.eclipse.mat.ui.help/tasks/comparingdata.dita

<?xml version="1.0" encoding="UTF-8"?>
<!--
    Copyright (c) 2011 SAP AG and IBM Corporation.
    All rights reserved. This program and the accompanying materials
    are made available under the terms of the Eclipse Public License v1.0
    which accompanies this distribution, and is available at
    http://www.eclipse.org/legal/epl-v10.html
   
    Contributors:
        SAP AG - initial documentation
        IBM Corporation - documentation on IBM specific dumps
 -->
<!DOCTYPE task PUBLIC "-//OASIS//DTD DITA Task//EN" "task.dtd" >
<task id="task_comparingdata" xml:lang="en-us">
	<title>Comparing Objects</title>
	<prolog>
		<copyright>
			<copyryear year=""></copyryear>
			<copyrholder>
				Copyright (c) 2011, 2012 SAP AG and IBM Corporation.
			    All rights reserved. This program and the accompanying materials
			    are made available under the terms of the Eclipse Public License v1.0
			    which accompanies this distribution, and is available at
			    http://www.eclipse.org/legal/epl-v10.html
			</copyrholder>
		</copyright>
	</prolog>

	<taskbody>
		<context>
		    <p><b>Introduction</b></p>
			<p>
		    Before looking into the capabilities that Memory Analyzer offers in the area of comparison
		    let us make some explanation. Object IDs which are provided in the heap dump formats supported by MAT
		    are just the addresses at which the objects are located. As objects are often moved and 
		    reordered by the JVM during a GC these addressed change. Therefore they cannot be used to compare the objects.
		    This basically means that if one compares two different heap dumps (although from the same process) it is not
		    possible to point to the concrete objects different between the two heap dumps.
		    However, one can still perform comparison on the aggregated results (e.g. the class histogram)
		    and analyze how the amount of object and the memory they take has changed.
			</p>

			<p>
				Memory Analyzer offers the possibility to compare not only the global class histograms of two different
				heap dumps, but an arbitrary number of table-formatted results - for example the 
				retained sets of three different objects. It doesn't matter if the tables which 
				are compared come from one and the same or different heap dumps.
			</p>
			<p>
				This means that one has the possibility to do things like:
				<ul>
				<li>Compare the retained set of a specific package across several heap dumps</li>
				<li>Compare how the retained sets for application objects A1, A2 and A3 (all in the 
				same heap dump) differ from each other</li>
				</ul> 
			</p>
			<p>				
				Here is a quick description how to compare several retained set tables.
			</p>
		</context>
				
		<steps>
			<step>
			<cmd>
				<b>Move all tables to be compared to the Compare Basket</b>
			</cmd>
			<info>
			<p>
				All queries which are executed in Memory Analyzer can be seen in the Navigation History View.
				From this view one can add the results to be compared to the Compare Basket. 
				The Navigation History is still per heap dump, therefore if one wants to compare 
				tables from different heap dumps then they have to be added one by one.
				Multiple tables from one heap dump can be added at a time.
			</p>
			<p>Trees can be compared as well, though they are converted to tables
			for the purposes of the comparison. 
			<xref href="queryingheapobjects.dita">Object Query Language (OQL)</xref> results
			can also be compared, though only the last result from each OQL editor can be 
			added to the Compare Basket. If another query is issued then the previous result
			will be removed from the basket. If two OQL results need to be compared then
			two OQL editors should be opened.</p>
			</info>			
			<stepresult>
				<image href="../mimes/nn_add_to_compare_basket.png">
					<alt>Move from Navigation History to Compare Basket</alt>
				</image>
			</stepresult>
			</step>
			<step>
			<cmd>
				<b>Modify the order of the tables</b>
			</cmd>
			<info>
			<p>
				Using the tool bar of the Compare Basket one can modify the order in which tables 
				should be compared, i.e. select which result should be the baseline, which comes second, etc...
			</p>
			</info>
			<stepresult>
				<image href="../mimes/nn_compare_basket.png">
					<alt>Modify the Tables Order in the Compare Basket</alt>
				</image>
			</stepresult>
			</step>
			<step>
			<cmd>
				<b>Execute the comparison</b>
			</cmd>
			<choices>
			<choice>
				Once the preferred order is achieved just click the execute button...
			<p>
				<image href="../mimes/nn_click_compare.png">
					<alt>Click Compare</alt>
				</image>
			</p>
			</choice>
			<choice>
				or to compare a subset of the tables, bring up the context menu on selected table entries.
				When comparing tables from one and the same heap dump, it is now possible
			to perform different set operations on the comparison result.
				<image href="../mimes/compare_basket_context_menu.png">
					<alt>Context menu in the Compare Basket</alt>
				</image>
			</choice>
			</choices>
			<stepresult>
			<p>
				... and see the result.
			</p>
			<p>
				<image href="../mimes/nn_compared_tables_absolute.png">
					<alt>Comparison table</alt>
				</image>
			</p>
			</stepresult>
			</step>
			<step>
			<cmd>
				<b>Customize the displayed result</b>
			</cmd>
			<info>
			<p>
				By default the absolute values of all tables will be shown for every compared property,
				e.g. number of objects, shallow size, etc... One can now change between deltas and
				absolute values, as well as select which columns should be compared:
			</p>
			<p>
				<image href="../mimes/nn_select_compare_option.png">
					<alt>Switch between Absolute Values and Deltas</alt>
				</image>
				<image href="../mimes/nn_select_columns.png">
					<alt>Select Columns To Be Compared</alt>
				</image>				
			</p>
			</info>
			<stepresult>
			<p>
				<image href="../mimes/nn_compared_result_delta.png">
					<alt>Modified Comparison Result</alt>
				</image>
			</p>
			</stepresult>
			</step>
			<step>
			<cmd>
				<b>Context menu with set operations</b>
			</cmd>
			<info>
				<p>If the tables have been compared using <menucascade><uicontrol>Compare Tables with all set operations</uicontrol>
					</menucascade> then it is possible to perform different set operations on the comparison result.
				</p>
				<image href="../mimes/setops_menu.png">
					<alt>Set Operations context menu</alt>
				</image>
			</info>
			</step>
		</steps>
	</taskbody>
</task>