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From "Alan Gates (JIRA)" <j...@apache.org>
Subject [jira] Updated: (PIG-164) In scripts that create large groups pig runs out of memory
Date Thu, 20 Mar 2008 16:33:25 GMT

     [ https://issues.apache.org/jira/browse/PIG-164?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Alan Gates updated PIG-164:
---------------------------

    Status: Patch Available  (was: Open)

The attached patch addresses the issue by changing the memory manager to do some cleaning
of the list whenever a databag is registered.  

I tried two previous approaches that did not work.

First, I had the memory manager spin a separate thread that woke up every five seconds and
cleaned the list.  For reasons that are entirely unclear to me this solution ran out of memory
faster than before.

Second, I had the register call clean the entire list.  This proved to be far too expensive,
and slowed down performance by about 10x.  

So, in this final register begins searching at the head of list, cleaning any weak references
it can.  As soon as it encounters an entry in the list that is valid, it quits looking.  This
avoids long searches through the list when most of the entries are valid.  It rests on the
assumption that data bags generally live about the same amount of time, thus there won't be
a long lived databag at the head of the list blocking the cleaning of many stale references
later in the list.

> In scripts that create large groups pig runs out of memory
> ----------------------------------------------------------
>
>                 Key: PIG-164
>                 URL: https://issues.apache.org/jira/browse/PIG-164
>             Project: Pig
>          Issue Type: Bug
>          Components: impl
>    Affects Versions: 0.0.0
>            Reporter: Alan Gates
>            Assignee: Alan Gates
>
> Scripts that need to group large amounts of data, such as a group all with 20m records,
often die with errors indicating that no more memory can be allocated.  PIG-40 addressed this
somewhat, but not completely.  In fact, it appears that in some situations it made it worse.
 If a script creates many data bags it can now run out of memory tracking all those data bags
that it may need to spill even if none of those bags gets very large.
> The issue is that the fix to PIG-40 introduced a memory manager that has a LinkedList
of WeakReferences that it uses to track these data bags.  When it is told by the memory manager
to dump memory, it walks this LinkedList, cleaning any entries that have gone stale and dumping
any that are still valid.  The problem is that in a script that processes many rows, the LinkedList
itself grows very large, and becomes the cause of needing to dump memory.

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