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https://issues.apache.org/jira/browse/HIVE-3992?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Gopal V updated HIVE-3992:
--------------------------
Attachment: HIVE-3992.2.patch
Update patch to trunk.
Fix the test failure where overlapping splits of varying sizes over the same
file are read.
Fix thread-safety issues & GC overhead errors (for HBase use-case), with a
Synchronized WeakHashMap.
> Hive RCFile::sync(long) does a sub-sequence linear search for sync blocks
> -------------------------------------------------------------------------
>
> Key: HIVE-3992
> URL: https://issues.apache.org/jira/browse/HIVE-3992
> Project: Hive
> Issue Type: Bug
> Environment: Ubuntu x86_64/java-1.6/hadoop-2.0.3
> Reporter: Gopal V
> Assignee: Gopal V
> Attachments: HIVE-3992.2.patch, HIVE-3992.patch,
> select-join-limit.html
>
>
> The following function does some bad I/O
> {code}
> public synchronized void sync(long position) throws IOException {
> ...
> try {
> seek(position + 4); // skip escape
> in.readFully(syncCheck);
> int syncLen = sync.length;
> for (int i = 0; in.getPos() < end; i++) {
> int j = 0;
> for (; j < syncLen; j++) {
> if (sync[j] != syncCheck[(i + j) % syncLen]) {
> break;
> }
> }
> if (j == syncLen) {
> in.seek(in.getPos() - SYNC_SIZE); // position before
> // sync
> return;
> }
> syncCheck[i % syncLen] = in.readByte();
> }
> }
> ...
> }
> {code}
> This causes a rather large number of readByte() calls which are passed onto a
> ByteBuffer via a single byte array.
> This results in rather a large amount of CPU being burnt in a the linear
> search for the sync pattern in the input RCFile (upto 92% for a skewed
> example - a trivial map-join + limit 100).
> This behaviour should be avoided at best or at least replaced by a rolling
> hash for efficient comparison, since it has a known byte-width of 16 bytes.
> Attached the stack trace from a Yourkit profile.
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