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From "Shao, Saisai" <>
Subject RE: MEMORY_ONLY_SER question
Date Wed, 05 Nov 2014 06:29:20 GMT
From my understanding, the Spark code use Kryo as a streaming manner for RDD partitions, the
deserialization comes with iteration to move forward. But the internal thing of Kryo to deserialize
all the object once or incrementally is actually a behavior of Kryo, I guess Kyro will not
deserialize the objects once for all.


From: Mohit Jaggi []
Sent: Wednesday, November 05, 2014 2:01 PM
To: Tathagata Das
Subject: Re: MEMORY_ONLY_SER question

I used the word "streaming" but I did not mean to refer to spark streaming. I meant if a partition
containing 10 objects was kryo-serialized into a single buffer, then in a mapPartitions()
call, as I call 10 times to access these objects one at a time, does the deserialization
a) once to get all 10 objects,
b) 10 times "incrementally" to get an object at a time, or
c) 10 times to get 10 objects and discard the "wrong" 9 objects [ i doubt this would a design
anyone would have adopted ]
I think your answer is option (a) and you refered to Spark streaming to indicate that there
is no difference in its behavior from spark core...right?

If it is indeed option (a), I am happy with it and don't need to customize. If it is (b),
I would like to have (a) instead.

I am also wondering if kryo is good at compression of strings and numbers. Often I have the
data type as "Double" but it could be encoded in much fewer bits.

On Tue, Nov 4, 2014 at 1:02 PM, Tathagata Das <<>>
It it deserialized in a streaming manner as the iterator moves over the partition. This is
a functionality of core Spark, and Spark Streaming just uses it as is.
What do you want to customize it to?

On Tue, Nov 4, 2014 at 9:22 AM, Mohit Jaggi <<>>
If I have an RDD persisted in MEMORY_ONLY_SER mode and then it is needed for a transformation/action
later, is the whole partition of the RDD deserialized into Java objects first before my transform/action
code works on it? Or is it deserialized in a streaming manner as the iterator moves over the
partition? Is this behavior customizable? I generally use the Kryo serializer.


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