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https://issues.apache.org/jira/browse/HADOOP-5589?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12689884#action_12689884
]
Jingkei Ly commented on HADOOP-5589:
------------------------------------
The example below demonstrates the current 64-value limit in TupleWritable:
{code}
Text emptyText = new Text("Should not be set written");
public void testTupleBoundarySuccess() throws Exception {
Writable[] values = new Writable[64];
Arrays.fill(values,emptyText);
values[63] = new Text("Should be the only value set written");
TupleWritable tuple = new TupleWritable(values);
tuple.setWritten(63);
for (int pos=0; pos<tuple.size();pos++) {
boolean has = tuple.has(pos);
if (pos == 63) {
assertTrue(has);
}
else {
assertFalse("Tuple position is incorrectly labelled as set: " + pos,
has);
}
}
}
public void testTupleBoundaryFailure() throws Exception {
Writable[] values = new Writable[65];
Arrays.fill(values,emptyText);
values[64] = new Text("Should be the only value set written");
TupleWritable tuple = new TupleWritable(values);
tuple.setWritten(64);
for (int pos=0; pos<tuple.size();pos++) {
boolean has = tuple.has(pos);
if (pos == 64) {
assertTrue(has);
}
else {
assertFalse("Tuple position is incorrectly labelled as set: " + pos,
has);
}
}
}
{code}
> TupleWritable: Lift implicit limit on the number of values that can be stored
> -----------------------------------------------------------------------------
>
> Key: HADOOP-5589
> URL: https://issues.apache.org/jira/browse/HADOOP-5589
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Affects Versions: 0.21.0
> Reporter: Jingkei Ly
>
> TupleWritable uses an instance field of the primitive type, long, which I
> presume is so that it can quickly determine if a position has been written to
> in its array of Writables (by using bit-shifting operations on the long
> field). The problem with this is that it implies that there is a maximum
> limit of 64 values you can store in a TupleWritable.
> An example of a use-case where I think this would be a problem is if you had
> two MR jobs with over 64 reduces tasks and you wanted to join the outputs
> with CompositeInputFormat - this will probably cause unexpected results in
> the current scheme.
> At the very least, the 64-value limit should be documented in TupleWritable.
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