Parth Chandra created DRILL-1504:
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Summary: Enabling fragment memory limit causes out of memory error
Key: DRILL-1504
URL: https://issues.apache.org/jira/browse/DRILL-1504
Project: Apache Drill
Issue Type: Bug
Components: Execution - Flow
Affects Versions: 0.6.0
Environment:
Reporter: Parth Chandra
Fix For: 0.7.0
When fragment memory limit is enabled, running a query with a large number of
fragments hits the fragment memory limit after being run a few times.
It appears there are two problems - 1) At the end of the query, the drillbit
does not reset the fragment limit to the amount before the query was run, and
2) the fragment limit seems to be smaller than expected.
The cause seems to be the following -
When a drillbit receives a request for a fragmentRecordBatch, the BitServer
threads create a NonRootFragmentManager object each, corresponding to the
FragmentHandle. Only one of the NonRootFragmentManager objects is actually
used, the others are discarded and garbage collected.
However, when fragment memory limit is enabled, the Allocator corresponding to
each of these nonRootFragmentManager objects registers the corresponding
FragmentContext with the top level allocator which then uses this information
to recalculate the fragment limit.
This has two effects - 1) the top level allocator counts more fragments because
it counts each fragment multiple times. 2) The top level allocator keeps a
reference to the fragment context which prevents the object from being garbage
collected. Worse, since no code actually 'closes' the fragment context, these
objects remain registered with the top level allocator across queries,
eventually causing an out of memory condition.
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