steveloughran commented on a change in pull request #2307:
URL: https://github.com/apache/hadoop/pull/2307#discussion_r490146585
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File path:
hadoop-tools/hadoop-azure/src/main/java/org/apache/hadoop/fs/azurebfs/services/AbfsInputStream.java
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@@ -180,9 +205,13 @@ private int readOneBlock(final byte[] b, final int off,
final int len) throws IO
// Enable readAhead when reading sequentially
if (-1 == fCursorAfterLastRead || fCursorAfterLastRead == fCursor ||
b.length >= bufferSize) {
+ LOG.debug("Sequential read with read ahead size of {}", bufferSize);
bytesRead = readInternal(fCursor, buffer, 0, bufferSize, false);
} else {
- bytesRead = readInternal(fCursor, buffer, 0, b.length, true);
+ // Enabling read ahead for random reads as well to reduce number of
remote calls.
+ int lengthWithReadAhead = Math.min(b.length + readAheadRange,
bufferSize);
+ LOG.debug("Random read with read ahead size of {}",
lengthWithReadAhead);
+ bytesRead = readInternal(fCursor, buffer, 0, lengthWithReadAhead,
true);
Review comment:
Based on the S3A experience (which didn't always read into a buffer,
BTW), the "penalty" of having a large readahead range is there is more data to
drain when you want to cancel the read (ie. a seek out of range).
That code does the draining in the active thread. If that were to be done in
a background thread, the penalty of a larger readahead would be less, as you
would only see a delay from the draining if there were no free HTTPS
connections in the pool. Setting up a new HTTPS connection is expensive though.
If there were no free HTTPS connections in the pool, you would be better off
draining the stream in the active thread. Maybe.
(Disclaimer: all my claims about cost of HTTPS are based on S3 +Java7/8, and
S3 is very slow to set up a connection. If the ADLS Gen2 store is faster to
negotiate then it becomes a lot more justifiable to drain in a separate thread)
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