[HBASE-14186] Read mvcc vlong optimization

• Type: Sub-task
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.0.0
• Component/s: Performance, Scanners
• Hadoop Flags: Reviewed
• Assignee: Anoop Sam John
• Reporter: Anoop Sam John
• Created: 04/Aug/15 17:39
• Updated: 15/Aug/15 00:25
• Resolved: 15/Aug/15 00:25

Description

for (int idx = 0; idx < remaining; idx++) {
  byte b = blockBuffer.getByteAfterPosition(offsetFromPos + idx);
  i = i << 8;
  i = i | (b & 0xFF);
}

Doing the read as in case of BIG_ENDIAN.
After HBASE-12600, we tend to keep the mvcc and so byte by byte read looks eating up lot of CPU time. (In my test HFileReaderImpl#_readMvccVersion comes on top in terms of hot methods). We can optimize here by reading 4 or 2 bytes in one shot when the length of the vlong is more than 4 bytes. We will in turn use UnsafeAccess methods which handles ENDIAN.