hadoop-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Bejoy KS" <bejoy.had...@gmail.com>
Subject Re: how to enhance job start up speed?
Date Mon, 13 Aug 2012 14:49:52 GMT
Hi Matthais

When an mapreduce program is being used there are some extra steps like checking for input
and output dir, calclulating input splits, JT assigning TT for executing the task etc.

If your file is non splittable , then one map task per file will be generated irrespective
of the number of hdfs blocks. Now some blocks will be in a different node than the node where
map task is executed so time will be spend here on the network transfer.

In your case MR would be a overhead as your file is non splittable hence no parallelism and
also there is an overhead of copying blocks to the map task node. 

Bejoy KS

Sent from handheld, please excuse typos.

-----Original Message-----
From: Matthias Kricke <matthias.mk.kricke@gmail.com>
Sender: matthias.zengler@gmail.com
Date: Mon, 13 Aug 2012 16:33:06 
To: <user@hadoop.apache.org>
Reply-To: user@hadoop.apache.org
Subject: Re: how to enhance job start up speed?

Ok, I try to clarify:

1) The worker is the logic inside my mapper and the same for both cases.
2) I have two cases. In the first one I use hadoop to execute my worker and
in a second one, I execute my worker without hadoop (simple read of the
   Now I measured, for both cases, the time the worker and
the surroundings need (so i have two values for each case). The worker took
the same time in both cases for the same input (this is expected). But the
surroundings took 17%  more time when using hadoop.
3) ~  3GB.

I want to know how to reduce this difference and where they come from.
I hope that helped? If not, feel free to ask again :)


P.S. just for your information, I did the same test with hypertable as
I got:
 * worker without anything: 15% overhead
 * worker with hadoop: 32% overhead
 * worker with hypertable: 53% overhead
Remark: overhead was measured in comparison to the worker. e.g. hypertable
uses 53% of the whole process time, while worker uses 47%.

2012/8/13 Bertrand Dechoux <dechouxb@gmail.com>

> I am not sure to understand and I guess I am not the only one.
> 1) What's a worker in your context? Only the logic inside your Mapper or
> something else?
> 2) You should clarify your cases. You seem to have two cases but both are
> in overhead so I am assuming there is a baseline? Hadoop vs sequential, so
> sequential is not Hadoop?
> 3) What are the size of the file?
> Bertrand
> On Mon, Aug 13, 2012 at 1:51 PM, Matthias Kricke <
> matthias.mk.kricke@gmail.com> wrote:
>> Hello all,
>> I'm using CDH3u3.
>> If I want to process one File, set to non splitable hadoop starts one
>> Mapper and no Reducer (thats ok for this test scenario). The Mapper
>> goes through a configuration step where some variables for the worker
>> inside the mapper are initialized.
>> Now the Mapper gives me K,V-pairs, which are lines of an input file. I
>> process the V with the worker.
>> When I compare the run time of hadoop to the run time of the same process
>> in sequentiell manner, I get:
>> worker time --> same in both cases
>> case: mapper --> overhead of ~32% to the worker process (same for bigger
>> chunk size)
>> case: sequentiell --> overhead of ~15% to the worker process
>> It shouldn't be that much slower, because of non splitable, the mapper
>> will be executed where the data is saved by HDFS, won't it?
>> Where did those 17% go? How to reduce this? Did hadoop needs the whole
>> time for reading or streaming the data out of HDFS?
>> I would appreciate your help,
>> Greetings
>> mk
> --
> Bertrand Dechoux

View raw message