Paul Rogers created DRILL-5013:
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Summary: Heap allocation, data copies in UDLE write path for
ExternalSortBatch
Key: DRILL-5013
URL: https://issues.apache.org/jira/browse/DRILL-5013
Project: Apache Drill
Issue Type: Improvement
Affects Versions: 1.8.0
Reporter: Paul Rogers
Priority: Minor
The ExternalSortBatch (ESB) uses spill-to-disk to sort a large collection of
records within a limited memory footprint.
As part of writing data to disk, ESB writes each of a target byte buffer to
disk. Since the vector is stored in direct memory (not visible to an output
stream), the code path first makes a temporary on-heap copy.
In particular the code in `io.netty.buffer.PooledUnsafeDirectByteBuf` does the
following:
{code}
@Override
public ByteBuf getBytes(int index, OutputStream out, int length) throws
IOException {
checkIndex(index, length);
if (length != 0) {
byte[] tmp = new byte[length];
PlatformDependent.copyMemory(addr(index), tmp, 0, length);
out.write(tmp);
}
return this;
}
{code}
The result is that we 1) create a large number of on-heap objects, and 2) copy
the data twice: once from direct memory to the tmp buffer, and from the tmp
buffer into the output stream's own buffer.
Two optimizations are possible:
1. Copy the data byte-by-byte from the direct memory buffer to the output
stream, or
2. Reuse the same tmp buffer across vector writes.
Since the code is in Netty, if we do either of the above, we'd have to write
our own "getBytes" (misnomer, really write bytes) method.
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