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https://issues.apache.org/jira/browse/HDFS-6621?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14124812#comment-14124812
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Yongjun Zhang commented on HDFS-6621:
-------------------------------------
Hi [~ravwojdyla],
I studied it a bit more, and it seems to me that on top of the changes you
made, we need to replace the {{Dispather.this}} in the following code with
{{this}},
{code}
try {
synchronized (Dispatcher.this) {
Dispatcher.this.wait(1000); // wait for targets/sources to be idle
}
} catch (InterruptedException ignored) {
}
{code}
so to make the scheduling threads with all five transfer threads
occupied/unfinished block on its {{source}}, then later if one transfer thread
finishes, it would notify this blocked scheduling thread (by your change for
problem 2) that a slot is available now.
If this makes sense to you, would please try it out with the testing you have
done?
Again, the first problem seems to be important to fix, but I don't know how
important the second one is (see question asked in my last comment). If the fix
of problem 1 is good enough, then we can go with it alone. Otherwise, my above
suggested change can be explored.
Would you please comment?
Thanks a lot.
> Hadoop Balancer prematurely exits iterations
> --------------------------------------------
>
> Key: HDFS-6621
> URL: https://issues.apache.org/jira/browse/HDFS-6621
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: balancer
> Affects Versions: 2.2.0, 2.4.0
> Environment: Red Hat Enterprise Linux Server release 5.8 with Hadoop
> 2.4.0
> Reporter: Benjamin Bowman
> Labels: balancer
> Attachments: HDFS-6621.patch, HDFS-6621.patch_2
>
>
> I have been having an issue with the balancing being too slow. The issue was
> not with the speed with which blocks were moved, but rather the balancer
> would prematurely exit out of it's balancing iterations. It would move ~10
> blocks or 100 MB then exit the current iteration (in which it said it was
> planning on moving about 10 GB).
> I looked in the Balancer.java code and believe I found and solved the issue.
> In the dispatchBlocks() function there is a variable,
> "noPendingBlockIteration", which counts the number of iterations in which a
> pending block to move cannot be found. Once this number gets to 5, the
> balancer exits the overall balancing iteration. I believe the desired
> functionality is 5 consecutive no pending block iterations - however this
> variable is never reset to 0 upon block moves. So once this number reaches 5
> - even if there have been thousands of blocks moved in between these no
> pending block iterations - the overall balancing iteration will prematurely
> end.
> The fix I applied was to set noPendingBlockIteration = 0 when a pending block
> is found and scheduled. In this way, my iterations do not prematurely exit
> unless there is 5 consecutive no pending block iterations. Below is a copy
> of my dispatchBlocks() function with the change I made.
> {code}
> private void dispatchBlocks() {
> long startTime = Time.now();
> long scheduledSize = getScheduledSize();
> this.blocksToReceive = 2*scheduledSize;
> boolean isTimeUp = false;
> int noPendingBlockIteration = 0;
> while(!isTimeUp && getScheduledSize()>0 &&
> (!srcBlockList.isEmpty() || blocksToReceive>0)) {
> PendingBlockMove pendingBlock = chooseNextBlockToMove();
> if (pendingBlock != null) {
> noPendingBlockIteration = 0;
> // move the block
> pendingBlock.scheduleBlockMove();
> continue;
> }
> /* Since we can not schedule any block to move,
> * filter any moved blocks from the source block list and
> * check if we should fetch more blocks from the namenode
> */
> filterMovedBlocks(); // filter already moved blocks
> if (shouldFetchMoreBlocks()) {
> // fetch new blocks
> try {
> blocksToReceive -= getBlockList();
> continue;
> } catch (IOException e) {
> LOG.warn("Exception while getting block list", e);
> return;
> }
> } else {
> // source node cannot find a pendingBlockToMove, iteration +1
> noPendingBlockIteration++;
> // in case no blocks can be moved for source node's task,
> // jump out of while-loop after 5 iterations.
> if (noPendingBlockIteration >= MAX_NO_PENDING_BLOCK_ITERATIONS) {
> setScheduledSize(0);
> }
> }
> // check if time is up or not
> if (Time.now()-startTime > MAX_ITERATION_TIME) {
> isTimeUp = true;
> continue;
> }
> /* Now we can not schedule any block to move and there are
> * no new blocks added to the source block list, so we wait.
> */
> try {
> synchronized(Balancer.this) {
> Balancer.this.wait(1000); // wait for targets/sources to be idle
> }
> } catch (InterruptedException ignored) {
> }
> }
> }
> }
> {code}
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