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https://issues.apache.org/jira/browse/HADOOP-3205?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12773191#action_12773191
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Raghu Angadi commented on HADOOP-3205:
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obviously, one buffer copy is not expected to consume more CPU than a CRC32
checksum (much less for checksum of small chunks like 512). I roughly esitmated
each buffer copy to take around 1/3rd of what CRC32 takes (ratio might be
larger with improved CRC32). Does it mean it is not worth fixing second or
third large CPU hogs on client? Of course when there is compression and other
higher processing is involved, even CRC32 wouldn't be the largest CPU hog.
We reduced CPU on DataNode while serving (HADOOP-2758, HADOOP-3164) mainly by
avoiding buffer copies (there is CRC involved). All the benchmarks there
measure CPU consumed based on actual CPU reported by the OS (not by a
profiler).. it is also essentially a 'dfs -cat'.
In your tests is it reading a dfs file? I used 'dfs -cat' extensively in
Datanode CPU benchmarks reported in the above Jiras.
bq. This seems like a fair amount of confusing complexity due to the buffer
management involved.
I am not so sure. But just not buffering at all might be good enough (the
smallest size would still be 512 bytes).
> FSInputChecker and FSOutputSummer should allow better access to user buffer
> ---------------------------------------------------------------------------
>
> Key: HADOOP-3205
> URL: https://issues.apache.org/jira/browse/HADOOP-3205
> Project: Hadoop Common
> Issue Type: Bug
> Components: fs
> Reporter: Raghu Angadi
> Assignee: Raghu Angadi
>
> Implementations of FSInputChecker and FSOutputSummer like DFS do not have
> access to full user buffer. At any time DFS can access only up to 512 bytes
> even though user usually reads with a much larger buffer (often controlled by
> io.file.buffer.size). This requires implementations to double buffer data if
> an implementation wants to read or write larger chunks of data from
> underlying storage.
> We could separate changes for FSInputChecker and FSOutputSummer into two
> separate jiras.
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