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From Stuart White <stuart.whi...@gmail.com>
Subject Re: Coordination between Mapper tasks
Date Fri, 20 Mar 2009 01:42:44 GMT
The nodes in my cluster have 4 cores & 4 GB RAM.  So, I've set
mapred.tasktracker.map.tasks.maximum to 3 (leaving 1 core for
"breathing room").

My process requires a large dictionary of terms (~ 2GB when loaded
into RAM).  The terms are looked-up very frequently, so I want the
terms memory-resident.

So, the problem is, I want 3 processes (to utilize CPU), but each
process requires ~2GB, but my nodes don't have enough memory to each
have their own copy of the 2GB of data.  So, I need to somehow share
the 2GB between the processes.

What I have currently implemented is a standalone RMI service that,
during startup, loads the 2GB dictionaries.  My mappers are simply RMI
clients that call this RMI service.

This works just fine.  The only problem is that my standalone RMI
service is totally "outside" Hadoop.  I have to ssh onto each of the
nodes, start/stop/reconfigure the services manually, etc...

So, I was thinking that, at job startup, the processes on each node
would (using ZooKeeper) elect a leader responsible for hosting the 2GB
dictionaries.  This process would load the dictionaries and share them
via RMI.  The other processes would recognize that another process on
the box is the leader, and they would act as RMI clients to that
process.

To make this work, I'm calling conf.setNumTasksToExecutePerJvm(-1) so
that Hadoop does not create new JVMs for each task.

Also note that the processes are "grouped" by node; that is, the
ZooKeeper path that I'll use for coordination will include the
hostname, so that only processes on the same node will compete for
leadership.

Anyway, in short, I was looking for a way to elect a leader process
per node responsible for hosting/sharing a large amount of
memory-resident data via RMI.

Hopefully that made sense...

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