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From "Hemanth Yamijala (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-5883) TaskMemoryMonitorThread might shoot down tasks even if their processes momentarily exceed the requested memory
Date Mon, 25 May 2009 04:45:45 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-5883?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Hemanth Yamijala updated HADOOP-5883:

    Attachment: HADOOP-5883.patch

The attached patch file incorporates the changes as mentioned in the earlier comment.

The key change was to determine processes older than a certain time. To do this, the process
tree class keeps track of an 'age' of the process - which is how many time the process tree
has seen a process with this PID. This count is updated every time the process tree is refreshed
- which is once every monitoring iteration. The monitoring thread can now ask for cumulative
virtual memory of processes over a certain 'age'. For the sake of simplicity, I've assumed
the monitoring interval determines how aged processes are.

It is possible to do something more sophisticated - for e.g. we could determine the walltime
of the process by making a system call. There doesn't seem to be a direct API for getting
the 'walltime' of a process. One hack would be to see the created time of the pid  directory
in /proc and then subtract it from timeofday each time. However, it seems like this could
be a costly operation, while not giving way too much more accuracy.

Summary of the changes:

- TaskMemoryManagerThread: changes to the logic to determine if a task is over limit. 
- ProcfsBasedProcessTree: introduces age for processes and updates them
- The rest of the changes were a lot to do with enabling fast unit tests to be written. I
think it is a good idea to move many of the tests for these two classes to use the testing
mechanism I'm using. But that's the focus of another JIRA.
- Introduced new tests for testing just these changes.

I suppose this patch will need merging with HADOOP-5881, which is likely to go in first. I
will update the patch with those changes once it's ready. This is just put up for early consumption.

Also missing is documentation updates for the new semantics of monitoring. Again, I will finish
that after the HADOOP-5881 merge.

> TaskMemoryMonitorThread might shoot down tasks even if their processes momentarily exceed
the requested memory
> --------------------------------------------------------------------------------------------------------------
>                 Key: HADOOP-5883
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5883
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>            Reporter: Hemanth Yamijala
>         Attachments: HADOOP-5883.patch
> Currently the TaskMemoryMonitorThread kills tasks as soon as it detects they are consuming
more memory than the max value specified. There are valid cases (see HADOOP-5059) where if
a program is executed from the task, it might momentarily occupy twice the amount of memory
for a short time. Ideally the monitoring thread should handle this case.

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