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https://issues.apache.org/jira/browse/HBASE-14186?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14654779#comment-14654779
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Anoop Sam John commented on HBASE-14186:
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bq.Is it possible, that we could come in here and there'd only be a short
amount to read so we'd skip the SIZEOF_INT parens? if so, the shift by 16 bits
in the second paren would be not needed (might not be a problem if left
shifting 0)
Yes shifting 0 is not a problem apart from this is an unwanted op. But I think
it is ok. The mvcc will be either 0 or some larger value with more than 4
bytes.
> Read mvcc vlong optimization
> ----------------------------
>
> Key: HBASE-14186
> URL: https://issues.apache.org/jira/browse/HBASE-14186
> Project: HBase
> Issue Type: Sub-task
> Components: Scanners
> Reporter: Anoop Sam John
> Assignee: Anoop Sam John
> Fix For: 2.0.0
>
> Attachments: HBASE-14186.patch
>
>
> {code}
> for (int idx = 0; idx < remaining; idx++) {
> byte b = blockBuffer.getByteAfterPosition(offsetFromPos + idx);
> i = i << 8;
> i = i | (b & 0xFF);
> }
> {code}
> 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.
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