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From Tom White <...@cloudera.com>
Subject Re: Too large class path for map reduce jobs
Date Thu, 07 Oct 2010 20:27:55 GMT
I wonder if there is a misunderstanding here - the problem is that the
classpath has too many classes on it (and clashes with user classes),
rather than it being a text string which is too long.

I would suggest that the technical discussion of how to fix this goes
onto the JIRA.

Cheers,
Tom

On Thu, Oct 7, 2010 at 1:23 AM, Alejandro Abdelnur <tucu@cloudera.com> wrote:
> well, if the issue is a too long classpath, the softlink thingy will give
> some room to breath as the total CP length will be much smaller.
>
> A
> On Thu, Oct 7, 2010 at 3:43 PM, Henning Blohm <henning.blohm@zfabrik.de>
> wrote:
>>
>> So that's actually another issue, right? Besides splitting the classpath
>> into those three groups, you want the TT to create soft-links on demand to
>> simplify the computation of classpath string. Is that right?
>>
>> But it's the TT that actually starts the job VM. Why does it matter what
>> the string actually looks like, as long as it has the right content?
>>
>> Thanks,
>>   Henning
>>
>> On Thu, 2010-10-07 at 13:22 +0800, Alejandro Abdelnur wrote:
>>
>> [sent too soon]
>>
>> The first CP shown is how it is today the CP of a task. If we change it
>> pick up all the job JARs from the current dir, then the classpath will be
>> much shorter (second CP shown). We can easily achieve this by soft-linking
>> the job JARs in the work dir of the task.
>>
>> Alejandro
>>
>> On Thu, Oct 7, 2010 at 1:02 PM, Alejandro Abdelnur <tucu@cloudera.com>
>> wrote:
>>
>> Fragmentation of Hadoop classpaths is another issue: hadoop should
>> differentiate the CP in 3:
>>
>> 1*client CP: what is needed to submit a job (only the nachos)
>>
>> 2*server CP (JT/NN/TT/DD): what is need to run the cluster (the whole
>> enchilada)
>>
>> 3*job CP: what is needed to run a job (some of the enchilada)
>>
>>
>> But i'm not trying to get into that here. What I'm suggesting is:
>>
>>
>>
>> -----
>>
>> # Hadoop JARs:
>>
>> /Users/tucu/dev-apps/hadoop/conf
>>
>>
>> /System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home/lib/tools.jar
>>
>> /Users/tucu/dev-apps/hadoop/bin/..
>>
>> /Users/tucu/dev-apps/hadoop/bin/../hadoop-core-0.20.3-CDH3-SNAPSHOT.jar
>>
>> /Users/tucu/dev-apps/hadoop/bin/../lib/aspectjrt-1.6.5.jar
>>
>> ..... (about 30 jars from hadoop lib/ )
>>
>> /Users/tucu/dev-apps/hadoop/bin/../lib/jsp-2.1/jsp-api-2.1.jar
>>
>> # Job JARs (for a job with only 2 JARs):
>>
>>
>> /Users/tucu/dev-apps/hadoop/dirs/mapred/taskTracker/distcache/-2707763075630339038_639898034_1993697040/localhost/user/tucu/oozie-tucu/0000003-101004184132247-oozie-tucu-W/java-node--java/java-launcher.jar
>>
>>
>> /Users/tucu/dev-apps/hadoop/dirs/mapred/taskTracker/distcache/3613772770922728555_-588832047_1993624983/localhost/user/tucu/examples/apps/java-main/lib/oozie-examples-2.2.1-CDH3B3-SNAPSHOT.jar
>>
>>
>> /Users/tucu/dev-apps/hadoop/dirs/mapred/taskTracker/tucu/jobcache/job_201010041326_0058/attempt_201010041326_0058_m_000000_0/work
>>
>> -----
>>
>>
>>
>> What I'm suggesting is that the later group, the job JARs to be
>> soft-linked (by the TT) into the working directory, then their classpath is
>> just:
>>
>> -----
>>
>> java-launcher.jar
>>
>> oozie-examples-2.2.1-CDH3B3-SNAPSHOT.jar
>>
>> .
>>
>> -----
>>
>>
>>
>>
>> Alejandro
>>
>> On Wed, Oct 6, 2010 at 7:57 PM, Henning Blohm <henning.blohm@zfabrik.de>
>> wrote:
>>
>> Hi Alejandro,
>>
>>    yes, it can of course be done right (sorry if my wording seemed to
>> imply otherwise). Just saying that I think that Hadoop M/R should not go
>> into that class loader / module separation business. It's one Job, one VM,
>> right? So the problem is to assign just the stuff needed to let the Job do
>> its business without becoming an obstacle.
>>
>>   Must admit I didn't understand your proposal 2. How would that remove
>> (e.g.) jetty libs from the job's classpath?
>>
>> Thanks,
>>   Henning
>>
>> Am Mittwoch, den 06.10.2010, 18:28 +0800 schrieb Alejandro Abdelnur:
>>
>> 1. Classloader business can be done right. Actually it could be done as
>> spec-ed for servlet web-apps.
>>
>>
>> 2. If the issue is strictly 'too large classpath', then a simpler solution
>> would be to sof-link all JARs to the current directory and create the
>> classpath with the JAR names only (no path). Note that the soft-linking
>> business is already supported by the DistributedCache. So the changes would
>> be mostly in the TT to create the JAR names only classpath before starting
>> the child.
>>
>>
>> Alejandro
>>
>>
>> On Wed, Oct 6, 2010 at 5:57 PM, Henning Blohm <henning.blohm@zfabrik.de>
>> wrote:
>>
>> Hi Tom,
>>
>>   that's exactly it. Thanks! I don't think that I can comment on the
>> issues in Jira so I will do it here.
>>
>>   Tricking with class paths and deviating from the default class loading
>> delegation has never been anything but a short term relieve. Fixing things
>> by imposing a "better" order of stuff on the class path will not work when
>> people do actually use child loaders (as the parent win) - like we do. Also
>> it may easily lead to very confusing situations because the former part of
>> the class path is not complete and gets other stuff from a latter part etc.
>> etc.... no good.
>>
>>   Child loaders are good for module separation but should not be used to
>> "hide" type visibiliy from the parent. Almost certainly leading to Class
>> Loader Contraint Violation - once you lose control (which is usually earlier
>> than expected).
>>
>>   The suggestion to reduce the Job class path to the required minimum is
>> the most practical approach. There is some gray area there of course and it
>> will not be feasible to reach the absolute minimal set of types there - but
>> something reasonable, i.e. the hadoop core that suffices to run the job.
>> Certainly jetty & co are not required for job execution (btw. I "hacked"
>> 0.20.2 to remove anything in "server/" from the classpath before setting the
>> job class path).
>>
>>   I would suggest to
>>
>>   a) introduce some HADOOP_JOB_CLASSPATH var that, if set, is the
>> additional classpath, added to the "core" classpath (as described above). If
>> not set, for compatibility, preserve today's behavior.
>>   b) not getting into custom child loaders for jobs as part of hadoop M/R.
>> It's non-trivial to get it right and feels to be beyond scope.
>>
>>   I wouldn't mind helping btw.
>>
>> Thanks,
>>   Henning
>>
>>
>>
>>
>> On Tue, 2010-10-05 at 15:59 -0700, Tom White wrote:
>>
>> Hi Henning,
>>
>> I don't know if you've seen
>> https://issues.apache.org/jira/browse/MAPREDUCE-1938 and
>> https://issues.apache.org/jira/browse/MAPREDUCE-1700 which have
>> discussion about this issue.
>>
>> Cheers
>> Tom
>>
>> On Fri, Sep 24, 2010 at 3:41 AM, Henning Blohm <henning.blohm@zfabrik.de>
>> wrote:
>> > Short update on the issue:
>> >
>> > I tried to find a way to separate class path configurations by modifying
>> > the
>> > scripts in HADOOP_HOME/bin but found that TaskRunner actually copies the
>> > class path setting from the parent process when starting a local task so
>> > that I do not see a way of having less on a job's classpath without
>> > modifying Hadoop.
>> >
>> > As that will present a real issue when running our jobs on Hadoop I
>> > would
>> > like to propose to change TaskRunner so that it sets a class path
>> > specifically for M/R tasks. That class path could be defined in the
>> > scipts
>> > (as for the other processes) using a particular environment variable
>> > (e.g.
>> > HADOOP_JOB_CLASSPATH). It could default to the current VM's class path,
>> > preserving today's behavior.
>> >
>> > Is it ok to enter this as an issue?
>> >
>> > Thanks,
>> >   Henning
>> >
>> >
>> > Am Freitag, den 17.09.2010, 16:01 +0000 schrieb Allen Wittenauer:
>> >
>> > On Sep 17, 2010, at 4:56 AM, Henning Blohm wrote:
>> >
>> >> When running map reduce tasks in Hadoop I run into classpath issues.
>> >> Contrary to previous posts, my problem is not that I am missing classes
>> >> on
>> >> the Task's class path (we have a perfect solution for that) but rather
>> >> find
>> >> too many (e.g. ECJ classes or jetty).
>> >
>> > The fact that you mention:
>> >
>> >> The libs in HADOOP_HOME/lib seem to contain everything needed to run
>> >> anything in Hadoop which is, I assume, much more than is needed to run
>> >> a map
>> >> reduce task.
>> >
>> > hints that your perfect solution is to throw all your custom stuff in
>> > lib.
>> > If so, that's a huge mistake.  Use distributed cache instead.
>> >
>>
>>
>>
>>
>>
>>
>>
>>
>>
>
>

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