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From Shi Yu <sh...@uchicago.edu>
Subject Re: large memory tasks
Date Wed, 15 Jun 2011 22:39:38 GMT
I had the same problem before, a big lookup table too large to load in 
memory.

I tried and compared the following approaches:  in-memory MySQL DB, a 
dedicated central memcache server, a dedicated central MongoDB server,  
local DB (each node has its own MongoDB server) model.

The local DB model is the most efficient one.  I believe dedicated 
server approach could get improved if the number of server is increased 
and distributed. I just tried single server.

But later I dropped out the lookup table approach. Instead, I attached 
the table information in the HDFS (which could be considered as an inner 
join DB process), which significantly increases the size of data sets 
but avoids the bottle neck of table look up. There is a trade-off, when 
no table looks up, the data to process is intensive (TB size). In 
contrast, a look-up table could save 90% of the data storage.

According to our experiments on a 30-node cluster, attaching information 
in HDFS is even 20%  faster than the local DB model. When attaching 
information in HDFS, it is also easier to ping-pong Map/Reduce 
configuration to further improve the efficiency.

Shi

On 6/15/2011 5:05 PM, GOEKE, MATTHEW (AG/1000) wrote:
> Is the lookup table constant across each of the tasks? You could try putting it into
memcached:
>
> http://hcil.cs.umd.edu/trs/2009-01/2009-01.pdf
>
> Matt
>
> -----Original Message-----
> From: Ian Upright [mailto:ian@upright.net]
> Sent: Wednesday, June 15, 2011 3:42 PM
> To: common-user@hadoop.apache.org
> Subject: large memory tasks
>
> Hello, I'm quite new to Hadoop, so I'd like to get an understanding of
> something.
>
> Lets say I have a task that requires 16gb of memory, in order to execute.
> Lets say hypothetically it's some sort of big lookuptable of sorts that
> needs that kind of memory.
>
> I could have 8 cores run the task in parallel (multithreaded), and all 8
> cores can share that 16gb lookup table.
>
> On another machine, I could have 4 cores run the same task, and they still
> share that same 16gb lookup table.
>
> Now, with my understanding of Hadoop, each task has it's own memory.
>
> So if I have 4 tasks that run on one machine, and 8 tasks on another, then
> the 4 tasks need a 64 GB machine, and the 8 tasks need a 128 GB machine, but
> really, lets say I only have two machines, one with 4 cores and one with 8,
> each machine only having 24 GB.
>
> How can the work be evenly distributed among these machines?  Am I missing
> something?  What other ways can this be configured such that this works
> properly?
>
> Thanks, Ian
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