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From Hemanth Yamijala <yhema...@yahoo-inc.com>
Subject Re: GridEngine module for Hadoop on Demand
Date Fri, 08 May 2009 04:50:30 GMT
Daniel Templeton wrote:
> Hi,
>
> I have a functioning module for Grid Engine for HoD, but some parts of 
> it are currently hard-coded to my workstation.  In cleaning up those 
> elements, I need some advice.  Hopefully this is the right forum.
>
> So, in the hodlib/NodePools/torque.py file, there's a runWorkers() 
> method.  In that method, it makes a single call to pbsdsh to start the 
> NameNode, DataNodes, JobTracker, and TaskTracker.  I know nada about 
> Torque, so please tell me if I'm interpreting this correctly.  It 
> would appear that the pbsdsh somehow reads out of the environment how 
> many hodring processes it should start up and executes them remotely, 
> and each hodring then figures out what service it should run.
>
Roughly right. In Torque, when a set of nodes are assigned to a job, the 
first node in that list is special (it's called mother superior - MS), 
the other nodes are called sisters. The job that's submitted to torque 
is a HOD process called 'ringmaster'. The ringmaster starts on the MS 
and invokes runWorkers which executes pbsdsh. AFAIK, pbsdsh reads the 
environment and gets a 'nodes' file that Torque writes out. This file 
contains all the sisters allocated for the job (including the MS). It 
executes the command passed to pbsdsh - another HOD process, called 
hodring - on all of these nodes. The Hodring processes work with the 
ringmaster and decide which service to run. In a sense the ringmaster 
coordinates which service to start where, and inform the hodring to 
start that service.
> In Grid Engine, the rough equivalent of pbsdsh is qrsh.  (I think.)  
> With qrsh, the master assigns the HoD job a set of nodes, and I then 
> have to step through that set of nodes and qrsh to each one to start 
> the hodring services.  As far as I can tell, the total number of 
> hodring services I need to start is 1 for the NameNode + 1 for the 
> JobTracker + n for the DataNodes + m for the TaskTrackers.  
HOD has a facility to use a HDFS service that's started outside of HOD. 
In that mode, it does not start NameNode or DataNodes. Also, the number 
of DataNodes always equals the number of TaskTrackers (if HDFS services 
are started with HOD).

> The thing that I'm not grokking is how the hodrings know what services 
> to start, and how I should be parceling them out across the nodes of 
> the cluster.  
This is decided by the ringmaster process. The logic is independent of 
the resource manager in use, and hence need not be worried about when 
porting to a new resource manager.

> Should I be making sure I have two hodrings per node, one for the 
> DataNode and one of the TaskTracker?  
No, a single hodring gets to start both the daemons.

> If I were to go start a dozen hodrings, one on each of a dozen 
> machines, would they work out among themselves how many should be 
> DataNodes and how many should be TaskTrackers? One more thing.  If the 
> above is on the mark, that means you're consuming a queue slot for 
> each DataNode unless you use an external hdfs service.  That seems 
> like a waste of cluster resources since slots tend to correspond more 
> to compute resources than I/O.  I have to wonder if it wouldn't be 
> more efficient from a cluster perspective to have each hodring start a 
> DataNode and a TaskTracker.  It would slightly oversubscribe that job 
> slot, but that may be better than grossly undersubscribing two.
>
Explained above.

Thanks
Hemanth

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