How. To. Debug. Map. Reduce. Programs - Hadoop Wiki. How to Debug Map/Reduce Programs. Debugging distributed programs is always difficult, because very few debuggers will let you connect to a remote program that wasn't run with the proper command line arguments. Start by getting everything running (likely on a small input) in the local runner. You do this by setting your job tracker to "local" in your config. The local runner can run under the debugger and runs on your development machine. A very quick and easy way to set this config variable is to include the following line just before you run the job: conf. You may also want to do this to make the input and output files be in the local file system rather than in the Hadoop distributed file system (HDFS): conf. You can also set these configuration parameters in hadoop- site. The configuration files hadoop- default. Run the small input on a 1 node cluster. This will smoke out all of the issues that happen with distribution and the "real" task runner, but you only have a single place to look at logs. Please note. This document refers to the 2.0 version of Apache httpd, which is no longer maintained. Upgrade, and refer to the current version of httpd instead, documented at: Current release version of Apache HTTP Server. Most useful are the task and job tracker logs. Make sure you are logging at the INFO level or you will miss clues like the output of your tasks. Run on a big cluster. Recently, I added the keep. This leaves "dead" files around that you can debug with. On the node with the failed task, go to the task tracker's local directory and cd to < local> /task. Tracker/< taskid> /work and run % hadoop org. Apache Debug Config Files For Counter-strikeIsolation. Runner ./job. This will run the failed task in a single jvm, which can be in the debugger, over precisely the same input. There is also a configuration variable (keep. Other than that, logging is your friend. Other Java Debugging hints. To print information about the state of the threads in a Java program including: Call stack Locks held Deadlocks Send a QUIT signal to the Java process: kill - QUIT < pid> The output is sent to stdout. If that doesn't work because the output is being sent to /dev/null, you can also use the commands: jps: List your java processes jstack: Get the call stack for a given Java process These commands were first included in Sun's Java 1. Setting Task Status. The map and reduce interfaces both include a parameter 'Reporter reporter'. The method Reporter. Status(String status) changes the displayed status of the map task and is visable on the jobtracker web page. This can be extremely useful to display debug information about the current record being handled, or setting certain debug flags about the status of the mapper. While running locally on a small data set can find many bugs, large data sets may contain pathological cases that are otherwise unexepcted. This method of debugging can help catch those cases. How to debug Hadoop Pipes programs. In order to debug Pipes programs you need to keep the downloaded commands. First, to keep the Task. Tracker from deleting the files when the task is finished, you need to set either keep. Second, your job should set hadoop. Job. Conf. This will cause all of the tasks in the job to write their command stream to a file in the working directory named downlink. This file will contain the Job. Conf, the task information, and the task input, so it may be large. But it provides enough information that your executable will run without any interaction with the framework. Third, go to the host where the problem task ran, go into the work directory and setenv hadoop. It will run as if the framework was feeding it commands and data and produce a output file downlink. Eventually, I'll probably make the downlink. Most problems however, will be pretty clear in the debugger or valgrind, even without looking at the generated data. The following sections are applicable only for Hadoop 0. Run a debug script when Task fails. When map/reduce task fails, there is a facility provided, via user- provided scripts, for doing post- processing on task logs i. The stdout and stderr of the user- provided debug script are printed on the diagnostics. These outputs are displayed on job UI on demand. For pipes, a default script is run which processes core dumps under gdb, prints stack trace and gives info about running threads. In the following sections we discuss how to submit debug script along with the job. We also discuss what the default behavior is. For submiting debug script, first it has to distributed. Supported. In the context of Apache HBase, /supported/ means that HBase is designed to work in the way described, and deviation from the defined behavior or functionality should be reported as a bug. Not Supported. In the. How to Debug Map/Reduce Programs. Debugging distributed programs is always difficult, because very few debuggers will let you connect to a remote program that wasn't run with the proper command line arguments. Start. CONFIG You may use a <Files.> directive in your httpd.conf Apache configuration file to make Apache::ASP start ticking. Configure the optional settings if you want, the defaults are fine to get started. The settings are. This sampler lets you send an HTTP/HTTPS request to a web server. It also lets you control whether or not JMeter parses HTML files for images and other embedded resources and sends HTTP requests to retrieve them. Apache Debug Config Files In HadoopThen the script has to supplied in Configuration. How to submit debug script file. To submit the debug script file, first put the file in dfs. The file can be distributed by setting the property "mapred. For more than one file, they can be added as comma seperated paths. The script file needs to be symlinked. This property can also be set by APIs Distributed. Cache. add. Cache. File(URI,conf) and Distributed. Cache. set. Cache. Files(URIs,conf) where URI is of the form "hdfs: //host: port/< absolutepath> #< script- name> ". For Streaming, the file can be added through command line option - cache. File. To create symlink for the file, the property "mapred. This can also be set by Distributed. Cache. create. Sym. Link. How to submit debug script. A quick way to submit debug script is to set values for the properties "mapred. These properties can also be set by APIs Job. Conf. set. Map. Debug. Script. Job. Conf. Reduce. Debug. Script. The script is given task's stdout, stderr, syslog, jobconf files as arguments. The debug command, run on the node where the map/reduce failed, is: $script $stdout $stderr $syslog $jobconf For streaming, debug script can be submitted with command- line options - mapdebug, - reducedebug for debugging mapper and reducer respectively. Pipes programs have the c++ program name as a fifth argument for the command. Thus for the pipes programs the command is $script $stdout $stderr $syslog $jobconf $program Here is an example on how to submit a script job. Conf. set. Map. Debug. Script("./myscript"). Distributed. Cache. Symlink(job. Conf). Distributed. Cache. Cache. File("/debug/scripts/myscript#myscript"); Default Behavior. The default behavior for failed map/reduce tasks is For Java programs: Stdout, stderr are shown on job UI. Stack trace is printed on diagnostics. For Pipes: Stdout, stderr are shown on the job UI. If the failed task has core file, Default gdb script is run which prints info abt threads: thread Id and function in which it was running when task failed. And prints stack trace where task has failed. For Streaming: Stdout, stderr are shown on the Job UI. The exception details are shown on task diagnostics.
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