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https://issues.apache.org/jira/browse/HADOOP-17250?focusedWorklogId=485651&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-485651
]
ASF GitHub Bot logged work on HADOOP-17250:
-------------------------------------------
Author: ASF GitHub Bot
Created on: 17/Sep/20 10:45
Start Date: 17/Sep/20 10:45
Worklog Time Spent: 10m
Work Description: steveloughran commented on a change in pull request
#2307:
URL: https://github.com/apache/hadoop/pull/2307#discussion_r490146585
##########
File path:
hadoop-tools/hadoop-azure/src/main/java/org/apache/hadoop/fs/azurebfs/services/AbfsInputStream.java
##########
@@ -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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Issue Time Tracking
-------------------
Worklog Id: (was: 485651)
Time Spent: 1h (was: 50m)
> ABFS: Allow random read sizes to be of buffer size
> --------------------------------------------------
>
> Key: HADOOP-17250
> URL: https://issues.apache.org/jira/browse/HADOOP-17250
> Project: Hadoop Common
> Issue Type: Sub-task
> Components: fs/azure
> Affects Versions: 3.3.0
> Reporter: Sneha Vijayarajan
> Assignee: Sneha Vijayarajan
> Priority: Major
> Labels: abfsactive, pull-request-available
> Time Spent: 1h
> Remaining Estimate: 0h
>
> ADLS Gen2/ABFS driver is optimized to read only the bytes that are requested
> for when the read pattern is random.
> It was observed in some spark jobs that though the reads are random, the next
> read doesn't skip by a lot and can be served by the earlier read if read was
> done in buffer size. As a result the job triggered a higher count of read
> calls and resulted in higher job runtime.
> When these jobs were run against Gen1 which always reads in buffer size , the
> jobs fared well.
> In this Jira we try to provide a control over config on random read to be of
> requested size or buffer size.
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