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