lirui-apache commented on code in PR #8463:
URL: https://github.com/apache/hadoop/pull/8463#discussion_r3167465565
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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:
Thanks for the review. I hit the issue when I was testing short circuit
read. If we call `BlockReaderLocal.read` with `tmp.remaining() == 0`, it will
hit the slow lane via `BlockReaderLocal::readWithBounceBuffer::fillDataBuf`,
which will actually allocate and read into the bounce buffer. But since user
buffer is filled, we cannot drain into it and the read data is thrown away and
wasted.
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