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From Sathwik B P <sathwik...@gmail.com>
Subject Re: Why LineRecordWriter.write(..) is synchronized
Date Sun, 11 Aug 2013 07:56:26 GMT
Hi Harsh,

Does it make any sense to keep the method in LRW still synchronized. Isn't
it creating unnecessary overhead for non multi threaded implementations.

regards,
sathwik

On Fri, Aug 9, 2013 at 7:16 AM, Harsh J <harsh@cloudera.com> wrote:

> I suppose I should have been clearer. There's no problem out of box if
> people stick to the libraries we offer :)
>
> Yes the LRW was marked synchronized at some point over 8 years ago [1]
> in support for multi-threaded maps, but the framework has changed much
> since then. The MultithreadedMapper/etc. API we offer now
> automatically shields the devs away from having to think of output
> thread safety [2].
>
> I can imagine there can only be a problem if a user writes their own
> unsafe multi threaded task. I suppose we could document that in the
> Mapper/MapRunner and Reducer APIs.
>
> [1] - http://svn.apache.org/viewvc?view=revision&revision=171186 -
> Commit added a synchronized to the write call.
> [2] - MultiThreadedMapper/etc. synchronize over the collector -
>
> http://svn.apache.org/viewvc/hadoop/common/trunk/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/lib/map/MultithreadedMapper.java?view=markup
>
> On Thu, Aug 8, 2013 at 7:52 PM, Azuryy Yu <azuryyyu@gmail.com> wrote:
> > sequence writer is also synchronized, I dont think this is bad.
> >
> > if you call HDFS api to write concurrently, then its necessary.
> >
> > On Aug 8, 2013 7:53 PM, "Jay Vyas" <jayunit100@gmail.com> wrote:
> >>
> >> Then is this a bug?  Synchronization in absence of any race condition is
> >> normally considered "bad".
> >>
> >> In any case id like to know why this writer is synchronized whereas the
> >> other one are not.. That is, I think, then point at issue: either other
> >> writers should be synchronized or else this one shouldn't be -
> consistency
> >> across the write implementations is probably desirable so that changes
> to
> >> output formats or record writers don't lead to bugs in multithreaded
> >> environments .
> >>
> >> On Aug 8, 2013, at 6:50 AM, Harsh J <harsh@cloudera.com> wrote:
> >>
> >> While we don't fork by default, we do provide a MultithreadedMapper
> >> implementation that would require such synchronization. But if you are
> >> asking is it necessary, then perhaps the answer is no.
> >>
> >> On Aug 8, 2013 3:43 PM, "Azuryy Yu" <azuryyyu@gmail.com> wrote:
> >>>
> >>> its not hadoop forked threads, we may create a line record writer, then
> >>> call this writer concurrently.
> >>>
> >>> On Aug 8, 2013 4:00 PM, "Sathwik B P" <sathwik.bp@gmail.com> wrote:
> >>>>
> >>>> Hi,
> >>>> Thanks for your reply.
> >>>> May I know where does hadoop fork multiple threads to use a single
> >>>> RecordWriter.
> >>>>
> >>>> regards,
> >>>> sathwik
> >>>>
> >>>> On Thu, Aug 8, 2013 at 7:06 AM, Azuryy Yu <azuryyyu@gmail.com>
wrote:
> >>>>>
> >>>>> because we may use multi-threads to write a single file.
> >>>>>
> >>>>> On Aug 8, 2013 2:54 PM, "Sathwik B P" <sathwik@apache.org>
wrote:
> >>>>>>
> >>>>>> Hi,
> >>>>>>
> >>>>>> LineRecordWriter.write(..) is synchronized. I did not find any
other
> >>>>>> RecordWriter implementations define the write as synchronized.
> >>>>>> Any specific reason for this.
> >>>>>>
> >>>>>> regards,
> >>>>>> sathwik
> >>>>
> >>>>
> >
>
>
>
> --
> Harsh J
>

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