I have 111 projects in my workspace and after Jboss Tools Richfaces installation Eclipse fails during build or validation with OutOfMemory.
I've checked jstack of my Eclipse process and see the following traces related to KB builder:

"Worker-2" prio=10 tid=0x0000000041154000 nid=0x5a9a waiting on condition [0x00007f589d10b000]
java.lang.Thread.State: RUNNABLE
at java.io.BufferedInputStream.<init>(BufferedInputStream.java:178)
at java.io.BufferedInputStream.<init>(BufferedInputStream.java:158)
at org.jboss.tools.common.model.filesystems.impl.JarAccess.getContent(JarAccess.java:201)
at org.jboss.tools.common.model.filesystems.impl.JarFolderImpl.createFileObject(JarFolderImpl.java:92)
at org.jboss.tools.common.model.filesystems.impl.JarFolderImpl.loadChildren(JarFolderImpl.java:74)
at org.jboss.tools.common.model.impl.RegularObjectImpl.getChildren(RegularObjectImpl.java:58)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parseInPackages(LibraryScanner.java:104)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parseInPackages(LibraryScanner.java:114)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parseInPackages(LibraryScanner.java:114)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parseInPackages(LibraryScanner.java:114)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parse(LibraryScanner.java:95)
at org.jboss.tools.jst.web.kb.internal.scanner.ClassPathMonitor.process(ClassPathMonitor.java:90)
at org.jboss.tools.jst.web.kb.internal.KbBuilder.build(KbBuilder.java:98)
at org.eclipse.core.internal.events.BuildManager$2.run(BuildManager.java:627)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:170)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:201)
at org.eclipse.core.internal.events.BuildManager$1.run(BuildManager.java:253)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:256)
at org.eclipse.core.internal.events.BuildManager.basicBuildLoop(BuildManager.java:309)
at org.eclipse.core.internal.events.BuildManager.build(BuildManager.java:341)
at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBuildJob.java:140)
at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJob.java:238)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:55)

and

"Text Viewer Hover Presenter" daemon prio=10 tid=0x00007f58a4cb2800 nid=0x5ad9 runnable [0x00007f588fd02000]
java.lang.Thread.State: RUNNABLE
at java.lang.String.intern(Native Method)
at org.jboss.tools.common.model.impl.XModelObjectImpl.set(XModelObjectImpl.java:135)
at org.jboss.tools.common.model.impl.RegularObjectImpl.set(RegularObjectImpl.java:103)
at org.jboss.tools.common.model.filesystems.impl.FileAnyImpl.set(FileAnyImpl.java:181)
at org.jboss.tools.common.meta.impl.adapters.XAdapterModelElement.setProperty(XAdapterModelElement.java:26)
at org.jboss.tools.common.model.impl.XModelObjectImpl.set_0(XModelObjectImpl.java:146)
at org.jboss.tools.common.model.impl.XModelImpl.createModelObject(XModelImpl.java:264)
at org.jboss.tools.common.model.filesystems.impl.JarFolderImpl.createFileObject(JarFolderImpl.java:96)
at org.jboss.tools.common.model.filesystems.impl.JarFolderImpl.loadChildren(JarFolderImpl.java:74)
at org.jboss.tools.common.model.impl.RegularObjectImpl.getChildren(RegularObjectImpl.java:58)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parseInPackages(LibraryScanner.java:104)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parseInPackages(LibraryScanner.java:114)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parseInPackages(LibraryScanner.java:114)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parseInPackages(LibraryScanner.java:114)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parseInPackages(LibraryScanner.java:114)
at org.jboss.tools.jst.web.kb.internal.scanner.LibraryScanner.parse(LibraryScanner.java:95)
at org.jboss.tools.jst.web.kb.internal.scanner.ClassPathMonitor.process(ClassPathMonitor.java:90)
at org.jboss.tools.jst.web.kb.internal.KbProject.load(KbProject.java:270)
at org.jboss.tools.jst.web.kb.internal.KbProject.resolveStorage(KbProject.java:228)
at org.jboss.tools.jst.web.kb.internal.KbProject.resolve(KbProject.java:238)
at org.jboss.tools.jst.web.kb.internal.KbProject.addKbProject(KbProject.java:162)
at org.jboss.tools.jst.web.kb.internal.scanner.ClassPathMonitor.validateProjectDependencies(ClassPathMonitor.java:121)
at org.jboss.tools.jst.web.kb.internal.KbProject.load(KbProject.java:255)
at org.jboss.tools.jst.web.kb.internal.KbProject.resolveStorage(KbProject.java:228)
at org.jboss.tools.jst.web.kb.internal.KbProject.resolve(KbProject.java:238)
at org.jboss.tools.jst.web.kb.internal.KbProject.addKbProject(KbProject.java:162)
at org.jboss.tools.jst.web.kb.internal.scanner.ClassPathMonitor.validateProjectDependencies(ClassPathMonitor.java:121)
at org.jboss.tools.jst.web.kb.internal.KbProject.load(KbProject.java:255)
at org.jboss.tools.jst.web.kb.internal.KbProject.resolveStorage(KbProject.java:228)
at org.jboss.tools.jst.web.kb.internal.KbProject.resolve(KbProject.java:238)
at org.jboss.tools.jst.web.kb.KbProjectFactory.getKbProject(KbProjectFactory.java:60)
at org.jboss.tools.jst.web.kb.taglib.TagLibraryManager.getLibraries(TagLibraryManager.java:35)
at org.jboss.tools.jst.web.kb.PageContextFactory.fillXMLNamespacesForNode(PageContextFactory.java:646)
at org.jboss.tools.jst.web.kb.PageContextFactory.fillContextForNode(PageContextFactory.java:461)
at org.jboss.tools.jst.web.kb.PageContextFactory.fillContextForChildNodes(PageContextFactory.java:445)
at org.jboss.tools.jst.web.kb.PageContextFactory.createPageContext(PageContextFactory.java:380)
at org.jboss.tools.jst.web.kb.PageContextFactory.createPageContext(PageContextFactory.java:179)
at org.jboss.tools.jst.web.kb.PageContextFactory.createPageContext(PageContextFactory.java:169)
at org.jboss.tools.jst.jsp.jspeditor.info.FaceletTagInfoHoverProcessor.computeHoverHelp(FaceletTagInfoHoverProcessor.java:62)
at org.eclipse.wst.xml.ui.internal.taginfo.XMLTagInfoHoverProcessor.getHoverInfo(XMLTagInfoHoverProcessor.java:257)
at org.jboss.tools.jst.jsp.jspeditor.info.ChainTextHover.getHoverInfo(ChainTextHover.java:106)
at org.eclipse.jface.text.TextViewerHoverManager$4.run(TextViewerHoverManager.java:168)

My Eclipse Vmargs are -XX:MaxPermSize=256m -Xmx1024m.
Disabling validation in Eclipse wont help.