Github user parthchandra commented on a diff in the pull request:
https://github.com/apache/drill/pull/611#discussion_r86446183
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/util/filereader/BufferedDirectBufInputStream.java
---
@@ -0,0 +1,460 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ * <p/>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p/>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.drill.exec.util.filereader;
+
+import com.google.common.base.Preconditions;
+import io.netty.buffer.DrillBuf;
+import org.apache.drill.common.config.DrillConfig;
+import org.apache.drill.exec.memory.BufferAllocator;
+import org.apache.drill.exec.memory.RootAllocatorFactory;
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.fs.ByteBufferReadable;
+import org.apache.hadoop.fs.FSDataInputStream;
+import org.apache.hadoop.fs.FileSystem;
+import org.apache.hadoop.fs.Path;
+import org.apache.parquet.hadoop.Footer;
+import org.apache.parquet.hadoop.ParquetFileReader;
+import org.apache.parquet.hadoop.metadata.BlockMetaData;
+import org.apache.parquet.hadoop.metadata.ColumnChunkMetaData;
+import org.apache.parquet.hadoop.util.CompatibilityUtil;
+
+import java.io.Closeable;
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.ByteBuffer;
+import java.util.List;
+
+/**
+ * <code>BufferedDirectBufInputStream</code> reads from the
+ * underlying <code>InputStream</code> in blocks of data, into an
+ * internal buffer. The internal buffer is a direct memory backed
+ * buffer. The implementation is similar to the
<code>BufferedInputStream</code>
+ * class except that the internal buffer is a Drillbuf and
+ * not a byte array. The mark and reset methods of the underlying
+ * <code>InputStream</code>are not supported.
--- End diff --
That's a question that's been asked of Drill before. The Parquet reader at
one point did read from the file system using byte arrays, but the overhead of
copying from byte arrays to direct memory was (it turned out) expensive. It
also increased the heap usage.
Given that, we switched to reading directly into DirectByteBuffers which
did improve performance somewhat, and definitely reduced the heap memory
requirements.
I couldn't think of any other way to read from the FS into direct memory
without writing this class. Using BufferedStream would have taken us back to
where we were before.
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