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From Enrico Minack <m...@Enrico.Minack.dev>
Subject Re: [pyspark2.4+] A lot of tasks failed, but job eventually completes
Date Mon, 06 Jan 2020 09:18:05 GMT
Note that repartitioning helps to increase the number of partitions (and 
hence to reduce the size of partitions and required executor memory), 
but subsequent transformations like join will repartition data again 
with the configured number of partitions 
(|spark.sql.shuffle.partitions|), virtually undoing the repartitioning, 
e.g.:

data                    // may have any number of partitions
   .repartition(1000)    // has 1000 partitions
   .join(table)          // has |spark.sql.shuffle.partitions|partitions

If you use RDDs, you need to configure |spark.default.parallelism| 
rather than |spark.sql.shuffle.partitions|.

Given you have 700GB of data, the default of 200 partitions mean that 
each partition is 3,5 GB (equivalent of input data) in size. Since 
increasing executor memory is limited by the available memory, executor 
memory does not scale for big data. Increasing the number of partitions 
is the natural way of scaling in Spark land.

Having hundreds of tasks that fail is an indication that you do not 
suffer from skewed data but from large partitions. Skewed data usually 
has a few tasks that keep failing.

It is easy to check for skewed data in the Spark UI. Open a stage that 
has failing tasks and look at the Summary Metrics, e.g.:
If the Max number of Shuffle Read Size is way higher than the 75th 
percentile, than this indicates a poor distribution of the data (or more 
precise the partitioning key) of this stage.

You can also sort the tasks by the "Shuffle Read Size / Records" column 
and see if numbers are evenly distributed (ideally).

I hope this helped.

Enrico



Am 06.01.20 um 06:27 schrieb hemant singh:
> You can try repartitioning the data, if it’s a skewed data then you 
> may need to salt the keys for better partitioning.
> Are you using a coalesce or any other fn which brings the data to 
> lesser nodes. Window function also incurs shuffling that could be an 
> issue.
>
> On Mon, 6 Jan 2020 at 9:49 AM, Rishi Shah <rishishah.star@gmail.com 
> <mailto:rishishah.star@gmail.com>> wrote:
>
>     Thanks Hemant, underlying data volume increased from 550GB to
>     690GB and now the same job doesn't succeed. I tried incrementing
>     executor memory to 20G as well, still fails. I am running this in
>     Databricks and start cluster with 20G assigned to
>     spark.executor.memory property.
>
>     Also some more information on the job, I have about 4 window
>     functions on this dataset before it gets written out.
>
>     Any other ideas?
>
>     Thanks,
>     -Shraddha
>
>     On Sun, Jan 5, 2020 at 11:06 PM hemant singh <hemant2184@gmail.com
>     <mailto:hemant2184@gmail.com>> wrote:
>
>         You can try increasing the executor memory, generally this
>         error comes when there is not enough memory in individual
>         executors.
>         Job is getting completed may be because when tasks are
>         re-scheduled it would be going through.
>
>         Thanks.
>
>         On Mon, 6 Jan 2020 at 5:47 AM, Rishi Shah
>         <rishishah.star@gmail.com <mailto:rishishah.star@gmail.com>>
>         wrote:
>
>             Hello All,
>
>             One of my jobs, keep getting into this situation where
>             100s of tasks keep failing with below error but job
>             eventually completes.
>
>             org.apache.spark.memory.SparkOutOfMemoryError: Unable to
>             acquire 16384 bytes of memory
>
>             Could someone advice?
>
>             -- 
>             Regards,
>
>             Rishi Shah
>
>
>
>     -- 
>     Regards,
>
>     Rishi Shah
>


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