A HashPartitioner will indeed partition based on the key, but you
cannot know on *which* node that key will appear. Again, the RDD
partitions will not necessarily be distributed evenly across your
nodes because of the greedy scheduling of the first wave of tasks,
particularly if those tasks have durations less than the initial
executor delay. I recommend you look at your logs to verify if this is
happening to you.
Mike
On 4/18/16, Anuj Kumar <anujs...@gmail.com> wrote:
> Good point Mike +1
>
> On Mon, Apr 18, 2016 at 9:47 AM, Mike Hynes <91m...@gmail.com> wrote:
>
>> When submitting a job with spark-submit, I've observed delays (up to
>> 1--2 seconds) for the executors to respond to the driver in order to
>> receive tasks in the first stage. The delay does not persist once the
>> executors have been synchronized.
>>
>> When the tasks are very short, as may be your case (relatively small
>> data and a simple map task like you have described), the 8 tasks in
>> your stage may be allocated to only 1 executor in 2 waves of 4, since
>> the second executor won't have responded to the master before the
>> first 4 tasks on the first executor have completed.
>>
>> To see if this is the cause in your particular case, you could try the
>> following to confirm:
>> 1. Examine the starting times of the tasks alongside their
>> executor
>> 2. Make a "dummy" stage execute before your real stages to
>> synchronize the executors by creating and materializing any random RDD
>> 3. Make the tasks longer, i.e. with some silly computational
>> work.
>>
>> Mike
>>
>>
>> On 4/17/16, Raghava Mutharaju <m.vijayaragh...@gmail.com> wrote:
>> > Yes its the same data.
>> >
>> > 1) The number of partitions are the same (8, which is an argument to
>> > the
>> > HashPartitioner). In the first case, these partitions are spread across
>> > both the worker nodes. In the second case, all the partitions are on
>> > the
>> > same node.
>> > 2) What resources would be of interest here? Scala shell takes the
>> default
>> > parameters since we use "bin/spark-shell --master <master-URL>" to run
>> the
>> > scala-shell. For the scala program, we do set some configuration
>> > options
>> > such as driver memory (12GB), parallelism is set to 8 and we use Kryo
>> > serializer.
>> >
>> > We are running this on Azure D3-v2 machines which have 4 cores and 14GB
>> > RAM.1 executor runs on each worker node. Following configuration
>> > options
>> > are set for the scala program -- perhaps we should move it to the spark
>> > config file.
>> >
>> > Driver memory and executor memory are set to 12GB
>> > parallelism is set to 8
>> > Kryo serializer is used
>> > Number of retainedJobs and retainedStages has been increased to check
>> them
>> > in the UI.
>> >
>> > What information regarding Spark Context would be of interest here?
>> >
>> > Regards,
>> > Raghava.
>> >
>> > On Sun, Apr 17, 2016 at 10:54 PM, Anuj Kumar <anujs...@gmail.com>
>> > wrote:
>> >
>> >> If the data file is same then it should have similar distribution of
>> >> keys.
>> >> Few queries-
>> >>
>> >> 1. Did you compare the number of partitions in both the cases?
>> >> 2. Did you compare the resource allocation for Spark Shell vs Scala
>> >> Program being submitted?
>> >>
>> >> Also, can you please share the details of Spark Context, Environment
>> >> and
>> >> Executors when you run via Scala program?
>> >>
>> >> On Mon, Apr 18, 2016 at 4:41 AM, Raghava Mutharaju <
>> >> m.vijayaragh...@gmail.com> wrote:
>> >>
>> >>> Hello All,
>> >>>
>> >>> We are using HashPartitioner in the following way on a 3 node cluster
>> (1
>> >>> master and 2 worker nodes).
>> >>>
>> >>> val u =
>> >>> sc.textFile("hdfs://x.x.x.x:8020/user/azureuser/s.txt").map[(Int,
>> >>> Int)](line => { line.split("\\|") match { case Array(x, y) =>
>> >>> (y.toInt,
>> >>> x.toInt) } }).partitionBy(new
>> HashPartitioner(8)).setName("u").persist()
>> >>>
>> >>> u.count()
>> >>>
>> >>> If we run this from the spark shell, the data (52 MB) is split across
>> >>> the
>> >>> two worker nodes. But if we put this in a scala program and run it,
>> then
>> >>> all the data goes to only one node. We have run it multiple times,
>> >>> but
>> >>> this
>> >>> behavior does not change. This seems strange.
>> >>>
>> >>> Is there some problem with the way we use HashPartitioner?
>> >>>
>> >>> Thanks in advance.
>> >>>
>> >>> Regards,
>> >>> Raghava.
>> >>>
>> >>
>> >>
>> >
>> >
>> > --
>> > Regards,
>> > Raghava
>> > http://raghavam.github.io
>> >
>>
>>
>> --
>> Thanks,
>> Mike
>>
>
--
Thanks,
Mike
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