Good morning,
I am a little confused, I have to say.
A summury of the project first: I want to examine how an Rtree on HDFS would speed up spatial queries like point/range queries, that normally target a very small part of the original input.
I have built my Rtree on HDFS, and now I need to answer queries using it. I thought I could make an MR Job that takes as input a text file where each line is a query (for example we have 20000 queries). To answer the queries efficiently, I need to check some information about the root nodes of the tree, which are stored in R files (R=the #reducers of the previous job). These files are small in size and are read from every mapper, thus the idea of distributed cache fits, right?
I have built an ArrayList during setup() to avoid opening all the files in distributed cache, and open only 3-4 of them for example. I agree, though, that opening and closing these files so many times is an important overhead. I think however, that opening these files from HDFS rather than distributed cache would be even worse, since the file accessing operations in HDFS are much more "expensive" than accessing files locally.
Thank you all for your response, I would be glad to have more feedback.
Sofia
________________________________
From: "GOEKE, MATTHEW (AG/1000)" <matthew.goeke@monsanto.com>
To: "common-user@hadoop.apache.org" <common-user@hadoop.apache.org>
Sent: Friday, August 12, 2011 7:05 PM
Subject: RE: Hadoop--store a sequence file in distributed cache?
Sofia correct me if I am wrong, but Mike I think this thread was about using the output of a previous job, in this case already in sequence file format, as in memory join data for another job.
Side note: does anyone know what the rule of thumb on file size is when using the distributed cache vs just reading from HDFS (join data not binary files)? I always thought that having a setup phase on a mapper read directly from HDFS was a asking for trouble and that you should always distribute to each node but I am hearing more and more people say to just read directly from HDFS for larger file sizes to avoid the IO cost of the distributed cache.
Matt
-----Original Message-----
From: Ian Michael Gumby
Sent: Friday, August 12, 2011 10:54 AM
To: common-user@hadoop.apache.org
Subject: RE: Hadoop--store a sequence file in distributed cache?
This whole thread doesn't make a lot of sense.
If your first m/r job creates the sequence files, which you then use as input files to your second job, you don't need to use distributed cache since the output of the first m/r job is going to be in HDFS.
(Dino is correct on that account.)
Sofia replied saying that she needed to open and close the sequence file to access the data in each Mapper.map() call.
Without knowing more about the specific app, Ashook is correct that you could read the file in Mapper.setup() and then access it in memory.
Joey is correct you can put anything in distributed cache, but you don't want to put an HDFS file in to distributed cache. Distributed cache is a tool for taking something from your job and distributing it to each job tracker as a local object. It does have a bit of overhead.
A better example is if you're distributing binary objects that you want on each node. A c++ .so file that you want to call from within your java m/r.
If you're not using all of the data in the sequence file, what about using HBase?
From: ashook@clearedgeit.com
To: common-user@hadoop.apache.org
Date: Fri, 12 Aug 2011 09:06:39 -0400
Subject: RE: Hadoop--store a sequence file in distributed cache?
If you are looking for performance gains, then possibly reading these files once during the setup() call in your Mapper and storing them in some data structure like a Map or a List will give you benefits. Having to open/close the files during each map call will have a lot of unneeded I/O.
You have to be conscious of your java heap size though since you are basically storing the files in RAM. If your files are a few MB in size as you said, then it shouldn't be a problem. If the amount of data you need to store won't fit, consider using HBase as a solution to get access to the data you need.
But as Joey said, you can put whatever you want in the Distributed Cache -- as long as you have a reader for it. You should have no problems using the SequenceFile.Reader.
-- Adam
-----Original Message-----
From: Joey Echeverria
Sent: Friday, August 12, 2011 6:28 AM
To: common-user@hadoop.apache.org; Sofia Georgiakaki
Subject: Re: Hadoop--store a sequence file in distributed cache?
You can use any kind of format for files in the distributed cache, so
yes you can use sequence files. They should be faster to parse than
most text formats.
-Joey
On Fri, Aug 12, 2011 at 4:56 AM, Sofia Georgiakaki
wrote:
Thank you for the reply!
In each map(), I need to open-read-close these files (more than 2 in the general case, and maybe up to 20 or more), in order to make some checks. Considering the huge amount of data in the input, making all these file operations on HDFS will kill the performance!!! So I think it would be better to store these files in distributed Cache, so that the whole process would be more efficient -I guess this is the point of using Distributed Cache in the first place!
My question is, if I can store sequence files in distributed Cache and handle them using e.g. the SequenceFile.Reader class, or if I should only keep regular text files in distributed Cache and handle them using the usual java API.
Thank you very much
Sofia
PS: The files have small size, a few KB to few MB maximum.
________________________________
From: Dino Kečo <dino.keco@gmail.com>
To: common-user@hadoop.apache.org; Sofia Georgiakaki <geosofie_tuc@yahoo.com>
Sent: Friday, August 12, 2011 11:30 AM
Subject: Re: Hadoop--store a sequence file in distributed cache?
Hi Sofia,
I assume that output of first job is stored on HDFS. In that case I would
directly read file from Mappers without using distributed cache. If you put
file into distributed cache that would add one more copy operation into your
process.
Thanks,
dino
On Fri, Aug 12, 2011 at 9:53 AM, Sofia Georgiakaki
wrote:
Good morning,
I would like to store some files in the distributed cache, in order to be
opened and read from the mappers.
The files are produced by an other Job and are sequence files.
I am not sure if that format is proper for the distributed cache, as the
files in distr.cache are stored and read locally. Should I change the format
of the files in the previous Job and make them Text Files maybe and read
them from the Distr.Cache using tha simple Java API?
Or can I still handle them with the usual way we use sequence files, even
if they reside in the local directory? Performance is extremely important
for my project, so I don't know what the best solution would be.
Thank you in advance,
Sofia Georgiakaki
--
Joseph Echeverria
Cloudera, Inc.
443.305.9434
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