After a while working with many tabs errors like this appear:

org.apache.wicket.protocol.http.PageExpiredException: Page expired.
at org.apache.wicket.request.handler.PageProvider.getPageInstance(PageProvider.java:171)
at org.apache.wicket.request.handler.ListenerInterfaceRequestHandler.getPage(ListenerInterfaceRequestHandler.java:89)
at org.apache.wicket.request.handler.ListenerInterfaceRequestHandler.respond(ListenerInterfaceRequestHandler.java:149)
at org.apache.wicket.request.cycle.RequestCycle$HandlerExecutor.respond(RequestCycle.java:719)
at org.apache.wicket.request.RequestHandlerStack.execute(RequestHandlerStack.java:63)
at org.apache.wicket.request.cycle.RequestCycle.processRequest(RequestCycle.java:210)
at org.apache.wicket.request.cycle.RequestCycle.processRequestAndDetach(RequestCycle.java:253)
at org.apache.wicket.protocol.http.WicketFilter.processRequest(WicketFilter.java:162)
at org.apache.wicket.protocol.http.WicketServlet.doGet(WicketServlet.java:137)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
at com.pmease.quickbuild.web.WebServlet.service(WebServlet.java:123)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.eclipse.equinox.http.helper.FilterServletAdaptor$FilterChainImpl.doFilter(FilterServletAdaptor.java:56)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserAgentFilter.java:78)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter.java:131)
at org.eclipse.equinox.http.helper.FilterServletAdaptor.service(FilterServletAdaptor.java:37)
at com.pmease.quickbuild.web.HttpServiceProvider$1$1.service(HttpServiceProvider.java:87)
at org.eclipse.equinox.http.servlet.internal.ServletRegistration.service(ServletRegistration.java:61)
at org.eclipse.equinox.http.servlet.internal.ProxyServlet.processAlias(ProxyServlet.java:126)
at org.eclipse.equinox.http.servlet.internal.ProxyServlet.service(ProxyServlet.java:76)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at org.eclipse.equinox.http.jetty.internal.HttpServerManager$InternalHttpServiceServlet.service(HttpServerManager.java:317)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:390)
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
at org.mortbay.jetty.Server.handle(Server.java:326)
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:924)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:549)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
at org.mortbay.jetty.bio.SocketConnector$Connection.run(SocketConnector.java:228)
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)

And an additional message:
The session is inactive for a long time and is expired to reduce server memory usage.
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standard diskdatastore and pagestore settings.
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We wouldn't like to get this message and server behavior at all. Does the server memory usage increase during session without page refreshes or even with navigating thru the application?
All the history is saved into wicket pages or is there upper limit?