[jira] [Created] (DRILL-5027) ExternalSortBatch can use excessive memory for very large queries

["Paul Rogers (JIRA)" <ji...@apache.org>]

Paul Rogers created DRILL-5027:
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             Summary: ExternalSortBatch can use excessive memory for very large queries
                 Key: DRILL-5027
                 URL: https://issues.apache.org/jira/browse/DRILL-5027
             Project: Apache Drill
          Issue Type: Bug
    Affects Versions: 1.8.0
            Reporter: Paul Rogers
            Priority: Minor


The {{ExternalSortBatch}} (ESB) operator sorts data while spilling to disk as needed to operate within a memory budget.

The sort happens in two phases:

1. Gather the incoming batches from the upstream operator, sort them, and spill to disk as needed.
2. Merge the "runs" spilled in step 1.

In most cases, the second step should run within the memory available for the first step (which is why severity is only Minor). However, if the query is exceptionally large, then the second step can use excessive memory.

Here is why.

* In step 1, we create a series of n spill files. Each file contains batches of some maximum size, say 20 MB.
* In step 2, we must simultaneously open all the spilled files and read the first batch into memory in preparation for merging.

Suppose that the query has 1 TB of data. Suppose that 10 GB of memory is available. The result will be 1 TB / 1 GB = 1000 spill files. Suppose each batch in each file is 20 MB. A single-pass merge will need 20 MB * 1000 = 20 GB to operate. But, we only have 10 GB.

The typical solution is to perform the merge in multiple phases, with each phase reading and respilling only enough runs that will fit in memory. In the above case, the first phase would make two iterations, merging 500 GB in each using 10 GB of memory. The second phase would make a single iteration to merge the two first phase files.

The result is much disk I/O, and a requirement for sufficient disk spaces to store two sets of spill files (one from phase i-1, another for phase i). But, the query will complete within the memory budgeted.



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