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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-13065) Add a new interface for retrieving FS and FC Statistics
Date Wed, 04 May 2016 11:10:13 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-13065?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15270485#comment-15270485
] 

Steve Loughran commented on HADOOP-13065:
-----------------------------------------

ALong has contention under load; it doesn't compile down to a simple x86 {{LOCK XADD $address,
value}}, which is all you need. The updater stuff is designed to get closer to this with something
eventually generated code like

{code}
address = &object + offset-to-field
LOCK XADD $address, value
{code}

(no, I can't code x86 ASM. I just spent lots of time staring at it trying to debug windows
C++ code)

The Java 9 stuff is actually intended to something really profound: provide the same operations
against arrays, so {{address = array + (aligned) offset}}, C's   *(array+offset) ++ were the
type of array some atomic long[].

The fencing stuff is just there to make people who think they understand memory models, CPU
and compiler re-ordering & the like write fast code. I've only ever seen anyone argue
for doing that in user level code once [Twitter: eventually consistent data structures|https://vimeo.com/43903960]
and their code only worked because they didn't understand that in Java 5+, volatile reads
are non-reorderable fences on all accesses. (that is: he gets the explanation of why things
work wrong). Nobody should be going near that in the Hadoop code at all.

> Add a new interface for retrieving FS and FC Statistics
> -------------------------------------------------------
>
>                 Key: HADOOP-13065
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13065
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: fs
>            Reporter: Ram Venkatesh
>            Assignee: Mingliang Liu
>         Attachments: HADOOP-13065-007.patch, HADOOP-13065.008.patch, HDFS-10175.000.patch,
HDFS-10175.001.patch, HDFS-10175.002.patch, HDFS-10175.003.patch, HDFS-10175.004.patch, HDFS-10175.005.patch,
HDFS-10175.006.patch, TestStatisticsOverhead.java
>
>
> Currently FileSystem.Statistics exposes the following statistics:
> BytesRead
> BytesWritten
> ReadOps
> LargeReadOps
> WriteOps
> These are in-turn exposed as job counters by MapReduce and other frameworks. There is
logic within DfsClient to map operations to these counters that can be confusing, for instance,
mkdirs counts as a writeOp.
> Proposed enhancement:
> Add a statistic for each DfsClient operation including create, append, createSymlink,
delete, exists, mkdirs, rename and expose them as new properties on the Statistics object.
The operation-specific counters can be used for analyzing the load imposed by a particular
job on HDFS. 
> For example, we can use them to identify jobs that end up creating a large number of
files.
> Once this information is available in the Statistics object, the app frameworks like
MapReduce can expose them as additional counters to be aggregated and recorded as part of
job summary.



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