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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4584) Slow generation of blockReport at DataNode causes delay of sending heartbeat to NameNode
Date Fri, 06 Feb 2009 07:33:59 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4584?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12671031#action_12671031
] 

Raghu Angadi commented on HADOOP-4584:
--------------------------------------


Synchronization is not correct. 

E.g. : The condition should be checked inside the lock that you wait on. It is not fatal in
this case only because it waits only for 1 sec, but still it is better not to write that way.
The block report thread should ideally do "{{wait(time till next report)}}", then the bug
shows up more.  A correct synchronization should work irrespective of wait time.

Similarly commandQueue and receviedBlockList sizes are checked outside the locks. We don't
need to synchronize on these two any more (since the patch uses common "dataAvailable" for
synchronization).

I don't think new class for CommandQueue is required. Also no need for conversion between
list and arrays etc. I would synchronize the following way :

{code}
// Block receiver thread :

   while (1) {
        cmd = null;
        synchronized (dataAvaliable) {
             cmd = commandQ.dequeue();
             if (cmd == null) {
                break;
         }
         processCmd(cmd);
         // similarly receivedBlockList. RPC should called outside lock.
    }

   // ...
   synchronized (dataAvailable) {
        if (cmdQ.size() == 0 || receivedBlockList.size() == 0) {
           dataAvailable.wait (time till next report);
       }
   }
                     
// heartBeat thread : 
   synchronize (dataAvailable) {
         cmdQ.addAll(cmds);
         dataAvailable.notify();
    }

// writers :
   synchronized (dataAvailable) {
        receivedBlockArray.add(block);
        dataAvailable.notify();
   }

// etc.. may be locking around delHints can also be removed.. though not related here.
    
{code}



> Slow generation of blockReport at DataNode causes delay of sending heartbeat to NameNode
> ----------------------------------------------------------------------------------------
>
>                 Key: HADOOP-4584
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4584
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>            Reporter: Hairong Kuang
>            Assignee: Suresh Srinivas
>             Fix For: 0.20.0
>
>         Attachments: 4584.patch, 4584.patch, 4584.patch
>
>
> sometimes due to disk or some other problems, datanode takes minutes or tens of minutes
to generate a block report. It causes the datanode not able to send heartbeat to NameNode
every 3 seconds. In the worst case, it makes NameNode to detect a lost heartbeat and wrongly
decide that the datanode is dead.
> It would be nice to have two threads instead. One thread is for scanning data directories
and generating block report, and executes the requests sent by NameNode; Another thread is
for sending heartbeats, block reports, and picking up the requests from NameNode. By having
these two threads, the sending of heartbeats will not get delayed by any slow block report
or slow execution of NameNode requests.

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