lirui-apache commented on code in PR #8463:
URL: https://github.com/apache/hadoop/pull/8463#discussion_r3256142315
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hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java:
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@@ -1230,7 +1230,9 @@ void actualGetFromOneDataNode(final DNAddrPair datanode,
final long startInBlk,
long beginReadMS = Time.monotonicNow();
int nread = 0;
int ret;
- while (true) {
+ // Stop once the slice is filled; an extra read with remaining()==0
+ // can trigger wasted slow-lane I/O in SCR.
+ while (tmp.hasRemaining()) {
Review Comment:
I was doing aligned positional reads with a DirectByteBuffer. Ideally I
should only hit the fast lane and avoid the bounce buffer. But I noticed some
cost from the slow lane and found the extra read. I believe the issue is pretty
easy to trigger, but whether it brings real perf regression depends on the
BlockReader implementations.
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