AppEngine manual scaling /_ah/start

[Alex Snsk via StackOverflow]

I have a task in AppEngine that needs more than 10 minutes to execute. I change the module to manual scaling B2 in appengine-web.xml as follows:

<manual-scaling>
    <instances>5</instances>
</manual-scaling>

Then when I request the URL the log fills with hundreds of following:

    F 15:31:23.421 2015-01-15  500       0 B   925ms /_ah/start
  0.1.0.3 - - [15/Jan/2015:06:31:23 -0800] "GET /_ah/start HTTP/1.1" 500 0 - - "3.1.bigcaty.catyv2.googleplex.com" ms=925 cpu_ms=1264 loading_request=1 exit_code=111 instance=3 app_engine_release=1.9.17
    W 15:31:23.403 /_ah/start
      java.lang.NullPointerException
        at com.google.api.client.extensions.appengine.auth.oauth2.AbstractAppEngineAuthorizationCodeServlet.getUserId(AbstractAppEngineAuthorizationCodeServlet.java:92)
        at com.google.api.client.extensions.servlet.auth.oauth2.AbstractAuthorizationCodeServlet.service(AbstractAuthorizationCodeServlet.java:122)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
        at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
        at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
        at com.google.apphosting.utils.servlet.ParseBlobUploadFilter.doFilter(ParseBlobUploadFilter.java:125)
        at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
        at com.google.apphosting.runtime.jetty.SaveSessionFilter.doFilter(SaveSessionFilter.java:35)
        at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
        at com.google.apphosting.utils.servlet.JdbcMySqlConnectionCleanupFilter.doFilter(JdbcMySqlConnectionCleanupFilter.java:60)
        at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
        at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
        at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
        at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
        at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
        at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
        at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
        at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
        at com.google.apphosting.runtime.jetty.AppVersionHandlerMap.handle(AppVersionHandlerMap.java:254)
        at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
        at org.mortbay.jetty.Server.handle(Server.java:326)
        at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
        at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
        at com.google.apphosting.runtime.jetty.RpcRequestParser.parseAvailable(RpcRequestParser.java:76)
        at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
        at com.google.apphosting.runtime.jetty.JettyServletEngineAdapter.serviceRequest(JettyServletEngineAdapter.java:146)
        at com.google.apphosting.runtime.JavaRuntime$RequestRunnable.run(JavaRuntime.java:484)
        at com.google.tracing.TraceContext$TraceContextRunnable.runInContext(TraceContext.java:438)
        at com.google.tracing.TraceContext$TraceContextRunnable$1.run(TraceContext.java:445)
        at com.google.tracing.CurrentContext.runInContext(CurrentContext.java:220)
        at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContextNoUnref(TraceContext.java:309)
        at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContext(TraceContext.java:301)
        at com.google.tracing.TraceContext$TraceContextRunnable.run(TraceContext.java:442)
        at com.google.apphosting.runtime.ThreadGroupPool$PoolEntry.run(ThreadGroupPool.java:251)
        at java.lang.Thread.run(Thread.java:724)

If I don't use manual scaling all works "perfectly". A request to a URL init the task that runs longer than 10 minutes, and it's killed. Any ideas why is this happening?

Thank you

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