Hadoop Tutorial 1.1 -- Generating Task Timelines

--D. Thiebaut 15:59, 18 April 2010 (UTC)

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This Hadoop tutorial shows how to generate Task Timelines similar to the ones generated by Yahoo in their report on the TeraSort experiment. See the page on Hadoop/MapReduce tutorials for additional tutorials.

Introduction

In May 2009 Yahoo announced it could sort a Petabyte of dat in 16.25 hours and a Terabyte of data in 62 seconds using Hadoop running on 3658 processors in the first case, and 1460 in the second case . In their report they show very convincing diagrams showing the evolution of the computation as a time-line of map, shuffle, sort, and reduce tasks as a function of time, an example of which is shown below.

The graph is generated by parsing one of the many logs generated by hadoop when a job is running, and is due to one of the authors of the Yahoo report cited above. It's original name is job_history_summary.py, and is available from here. We have renamed it generateTimeLine.py.

Generating the Log

First run a MapReduce program. We'll use the WordCount program of Tutorial #1.

• Run the word count program on your input directory where you have one or more text files containing large documents.

 hadoop jar /home/hadoop/352/dft/wordcount_counters/wordcount.jar org.myorg.WordCount dft6 dft6-output

• When the program is over, find the log of the job you just ran in the log-history (If you are the only user at the moment, it will be the last job in the list.):

 ls -ltr ~/hadoop/hadoop/logs/history/ 

 -rwxrwxrwx 1 hadoop hadoop 35578 2010-04-04 14:38 hadoop1_1270135155456_job_201004011119_0014_hadoop_wordcount

• Assume the file we're interested in is the 0014 one. Take a look at it

 less hadoop1_1270135155456_job_201004011119_0014_hadoop_wordcount

Note that it contains a wealth of information that is fairly cryptic and not necessarily easy to decypher.

• Check that the script generateTimeLine.py is installed on your system:

  which generateTimeLine.py          (if you get response to the command, you have it!)

If the script hasn't been installed yet, create a file in your path that contains the Yahoo script for parsing the log file. The script is also available here.

• Feed the log file above to the script:

 cat hadoop1_1270135155456_job_201004011119_0014_hadoop_wordcount | generateTileLine.py
 time maps shuffle merge reduce
 0 1 0 0 0
 1 1 0 0 0
 2 3 0 0 0
 3 4 0 0 0 
 4 4 0 0 0
 5 5 0 0 0
 6 5 0 0 0
 7 1 1 0 0
 8 0 1 0 0
 9 0 1 0 0
 10 0 1 0 0
 11 0 1 0 0
 12 0 1 0 0
 13 0 1 0 0
 14 0 1 0 0
 15 0 0 0 1
 16 0 0 0 1
 17 1 0 0 0

• Copy/Paste the output of the script into your favorite spreadsheet software and generate an Area graph for the data that you will have distributed in individual columns.

Note: If you are using Google Docs, the spreadsheet utility has no features (as of this writing) to split text into columns, so you might want to filter the output of generateTimeLine.py and feed it to generateColumns.py, as follows:

  cat hadoop1_1270135155456_job_201004011119_0014_hadoop_wordcount | generateTileLine.py | generateColumns.py

• Use the Area type graph. You should obtain something like this:

	Lab Experiment #1 Go ahead, and create a timeline of the tasks that ran during the execution of your MapReduce program.

Multiple Number of Reduce Tasks

The default is for 1 reduce task. If you want to use more, then use the -r switch when starting your hadoop job:

  hadoop jar /home/hadoop/352/dft/wordcount_countBuck/wordcount.jar org.myorg.WordCount -r 2 dft1 dft1-output

	Lab Experiment #2 Run your job again and this time try using 2 or more reduce tasks, and see their execution as reported in the timeline.

Another Example of timelines

This one shows the timeline of WordCount running on 4 text files (xml) of 180 MB each on a cluster of 6 single-core Linux servers. (data available here)

Question

Explain the dip in the middle of the Map tasks.

If you get strange results

If you get timelines that look strange, like this one:

you may want to run the following command on all the hadoop nodes:

sudo ntpdate ntp.ubuntu.com