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From "Paul Rogers (JIRA)" <j...@apache.org>
Subject [jira] [Created] (DRILL-5022) ExternalSortBatch sets two different limits for "copier" memory
Date Wed, 09 Nov 2016 02:07:58 GMT
Paul Rogers created DRILL-5022:
----------------------------------

             Summary: ExternalSortBatch sets two different limits for "copier" memory
                 Key: DRILL-5022
                 URL: https://issues.apache.org/jira/browse/DRILL-5022
             Project: Apache Drill
          Issue Type: Bug
    Affects Versions: 1.8.0
            Reporter: Paul Rogers
            Priority: Minor


The {{ExternalSortBatch}} (ESB) operator sorts rows and supports spilling to disk to operate
within a set memory budget.

A key step in disk-based sorting is to merge "runs" of previously-sorted records. ESB does
this with a class created from the {{PriorityQueueCopierTemplate}}, called the "copier" in
the code.

The sort runs are represented by record batches, each with an indirection vector (AKA {{SelectionVector}})
that point to the records in sort order.

The copier restructures the incoming runs: copying from the original batches (from positions
given by the indirection vector) into new output vectors in sorted order. To do this work,
the copier must allocate new vectors to hold the merged data. These vectors consume memory,
and must fit into the overall memory budget assigned to the ESB.

As it turns out, the ESB code has two conflicting ways of setting the limit. One is hard-coded:
{code}
  private static final int COPIER_BATCH_MEM_LIMIT = 256 * 1024;
{code}

The other comes from config parameters:
{code}
  public static final long INITIAL_ALLOCATION = 10_000_000;
  public static final long MAX_ALLOCATION = 20_000_000;

    copierAllocator = oAllocator.newChildAllocator(oAllocator.getName() + ":copier",
        PriorityQueueCopier.INITIAL_ALLOCATION, PriorityQueueCopier.MAX_ALLOCATION);
{code}

Strangely, the config parameters are used to set aside memory for the copier to use. But,
the {{COPIER_BATCH_MEM_LIMIT}} is used to determine how large of a merged batch to actually
create.

The result is that we set aside 10 MB of memory, but use only 256K of it, wasting 9 MB.

This ticket asks to:

* Determine the proper merged batch size.
* Use that limit to set the memory allocation for the copier.

Elsewhere in Drill batch sizes tend to be on the order of 32K records. In the ESB, the low
{{COPIER_BATCH_MEM_LIMIT}} tends to favor smaller batches: A test case has a row width of
114 bytes, and produces batches of just 2299 records. So, likely the proper choice is the
larger 10 MB memory allocator limit.



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