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From "Vivek Ratan (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-1462) Better progress reporting from a Task
Date Tue, 05 Jun 2007 15:17:25 GMT
Better progress reporting from a Task

                 Key: HADOOP-1462
                 URL: https://issues.apache.org/jira/browse/HADOOP-1462
             Project: Hadoop
          Issue Type: Improvement
          Components: mapred
            Reporter: Vivek Ratan

The Task code that reports progress updates has the following problems:
1. Some RPC calls are blocking. For example, in MapRunner::run(), the call to RecordReader::next()
can result in a blocking RPC call to the Task Tracker (TT) to report progress. 
2. Some RPC calls are unnecessary. The Ping thread pings the TT once every second, while we
also independently send progress updates every second. We don't, for example, need to ping
the TT right after we send the progress update. 
3. In some places, we spawn a thread to send progress updates (in MapOutputBuffer::collect(),
for example). If our code gets stuck, the thread will continue sending updates to the TT and
we will never be shut down. 

These issues, in some form or another, have been reported in HADOOP-1201 and HADOOP-1431.

I propose we make the following changes: 

1. In order to make the RPC calls non-blocking, we need a thread that calls TT. This thread,
to be created early on, will make sure we make the most appropriate RPCs. It will have access
to two flags: a progress flag that indicates that the Task has made progress since the last
RPC, and a keep_alive flag that indicates that we need to let the TT know that we're alive.
This thread will also handle pings. It's logic will be something like this: 

while (1) {
	if (progress_flag is set) {
		// report progress update
		if (failure), kill task;
		reset progress_flag; 
		reset keep_alive_flag; // calling progress() also indicates that we're alive
	else if (keep_alive_flag is set) {
		// let TT know we're alive
		umbilical::progress(same params as last time);
		if (failure), kill task;
		reset keep_alive_flag;
	else {
		// see if TT is alive
		if (failure), kill task;
	sleep (1 sec);

2. progress_flag and keep_alive_flag are set by the MapReduce code. Reporter::progress() (in
Task.java) sets keep_alive_flag while progress_flag is set whenever Task's taskProgress object
has any of its fields changed. 

3. We do away with Task::reportProgress() as this code is now handled in the Progress thread.
Wherever this method is called in our MapReduce kernel code, we should replace it either with
Reporter::progress() (if the intent was to let TT know that we're alive) or we simply remove
that call (if the intent was to transmit progress changes to the TT). 

4. TaskUmbilicalProtocol::progress() should return a boolean, and should return the same values
that TaskUmbilicalProtocol::ping() does. This will let the Task know whether its ID is known
to the TT. 

5. We no longer need to create a ping thread in TaskTracker::Child. However, we can perhaps
create the Progress thread in the same place the Ping thread was created. 

6. We will need to remove code that creates progress threads. This is in MapTask::MapOutputBuffer::collect(),
MapTask::MapOutputBuffer::flush(), and ReduceTask::ReduceCopier(), at the least. Instead,
we will need to add code that updates the progress or calls Reporter::progress(). Any of these
calls simply update flags. so there's not a lot of performance penalty (at worst, updating
progress_flag or keep_alive_flag may need to be done within a synchronized block, but even
that may not be necessary since the flags are just boolean values). As per HADOOP-1431, these
calls can be made through a ReportingComparator, or from within the generic BuferSorter, or
perhaps from some place better. 

I may have missed out on some details, but hopefully the overall idea is clear. Comments welcome.

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