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From "Amar Kamat (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-2782) Earlier key-value buffer from MapTask.java is still referenced even though its not required anymore.
Date Tue, 05 Feb 2008 14:42:08 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-2782?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Amar Kamat updated HADOOP-2782:
-------------------------------

    Priority: Minor  (was: Critical)

> Earlier key-value buffer from MapTask.java is still referenced even though its not required
anymore.
> ----------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-2782
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2782
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>            Reporter: Amar Kamat
>            Priority: Minor
>
> Consider the following events for a map task
> Before HADOOP-1965:
> || Stage || Description || Buffers used || Memory used||
> |Stage-1 | MapOutputBuffer simply collects | KeyVal1 (by collect) | io.sort.mb|
> |Stage-2 | KeyVal1 buffer is full and needs spilling so Sort-Spill starts | KeyVal1 (by
Sort-Spill) | io.sort.mb|
> |Stage-3 | Sort-Spill finished | KeyVal1 (referenced by comparator ) | io.sort.mb|
> |Stage-4 |  MapOutputBuffer starts collecting | KeyVal2(by collect) + KeyVal1(by comparator)
| 2*io.sort.mb|
> |Stage-5 | KeyVal2 buffer is full and needs spilling so Sort-Spill starts  | KeyVal2
(by Sort-Spill) | io.sort.mb|
> So for the time duration between Stage-4 and Stage-5 the memory used becomes {{2 * io.sort.mb}}
which can be totally avoided by removing the comparator's reference to the earlier key-val
buffer. So the maximum memory usage can be clamped to {{io.sort.mb}}
> After HADOOP-1965:
> || Stage || Description || Buffers used || Memory used ||
> |Stage-1 | MapOutputBuffer simply collects | KeyVal1 (by collect)| io.sort.mb/2|
> |Stage-2 | KeyVal1 buffer is full and needs spilling, so Sort-Spill starts in parallel
| KeyVal1 (by Sort-Spill) | io.sort.mb/2|
> |Stage-3 |  MapOutputBuffer simply collects + Sort-Spill | KeyVal2(by collect) + KeyVal1(by
Sort-Spill) | io.sort.mb|
> |Stage-4 | MapOutputBuffer simply collects + Sort-Spill finishes, Sort-Impl's are closed
but the comparators still hold the reference to KeyVal1 buffer | KeyVal2 (by collect) + KeyVal1
(referred by comparator) | io.sort.mb|
> |Stage-5 | KeyVal2 buffer is full and needs spilling, so Sort-Spill starts in parallel
| KeyVal2 (by Sort-Spill) | io.sort.mb/2|
> So for the time duration between Stage-4 and Stage-5 there is an unwanted reference to
the keyval buffer which prevents the GC from claiming it. However the maximum memory usage
will be {{io.sort.mb}}.

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