[jira] [Commented] (SPARK-21176) Master UI hangs with spark.ui.reverseProxy=true if the master node has many CPUs

["Ingo Schuster (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-21176?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16060553#comment-16060553 ] 

Ingo Schuster commented on SPARK-21176:
---------------------------------------

Best way I can see to control the number of selectors, is to overwrite the newHttpClient() method of ProxyServlet in our JettyUtils.scala.
It's a pitty that the constructor of ProxyServlet does not allow to overwrite the selector default.
Maybe something to clean up the code in future, I have opened an respective request  for Jetty: https://github.com/eclipse/jetty.project/issues/1643

> Master UI hangs with spark.ui.reverseProxy=true if the master node has many CPUs
> --------------------------------------------------------------------------------
>
>                 Key: SPARK-21176
>                 URL: https://issues.apache.org/jira/browse/SPARK-21176
>             Project: Spark
>          Issue Type: Bug
>          Components: Web UI
>    Affects Versions: 2.1.0, 2.1.1, 2.2.0, 2.2.1
>         Environment: ppc64le GNU/Linux, POWER8, only master node is reachable externally other nodes are in an internal network
>            Reporter: Ingo Schuster
>              Labels: network, web-ui
>
> In reverse proxy mode, Sparks exhausts the Jetty thread pool if the master node has too many cpus or the cluster has too many executers:
> For each ProxyServlet, Jetty creates Selector threads: minimum 4, maximum half the number of available CPUs:
> {{this(Math.max(1, Runtime.getRuntime().availableProcessors() / 2));}}
> (see https://github.com/eclipse/jetty.project/blob/0c8273f2ca1f9bf2064cd9c4c939d2546443f759/jetty-client/src/main/java/org/eclipse/jetty/client/http/HttpClientTransportOverHTTP.java)
> In reverse proxy mode, a proxy servlet is set up for each executor.
> I have a system with 7 executors and 88 CPUs on the master node. Jetty tries to instantiate 7*44 = 309 selector threads just for the reverse proxy servlets, but since the QueuedThreadPool is initialized with 200 threads by default, the UI gets stuck.
> I have patched JettyUtils.scala to extend the thread pool ( {{val pool = new QueuedThreadPool(400)}}). With this hack, the UI works.
> Obviously, the Jetty defaults are meant for a real web server. If that has 88 CPUs, you do certainly expect a lot of traffic.
> For the Spark admin UI however, there will rarely be concurrent accesses for the same application or the same executor.
> I therefore propose to dramatically reduce the number of selector threads that get instantiated - at least by default.
> I will propose a fix in a pull request.



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