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From Chris Teoh <>
Subject Re: Identify bottleneck
Date Thu, 19 Dec 2019 22:33:26 GMT
As far as I'm aware it isn't any better. The logic all gets processed by
the same engine so to confirm, compare the DAGs generated from both
approaches and see if they're identical.

On Fri, 20 Dec 2019, 8:56 am ayan guha, <> wrote:

> Quick question: Why is it better to use one sql vs multiple withColumn?
> isnt everything eventually rewritten by catalyst?
> On Wed, 18 Dec 2019 at 9:14 pm, Enrico Minack <>
> wrote:
>> How many withColumn statements do you have? Note that it is better to use
>> a single select, rather than lots of withColumn. This also makes drops
>> redundant.
>> Reading 25m CSV lines and writing to Parquet in 5 minutes on 32 cores is
>> really slow. Can you try this on a single machine, i.e. run wit "local[*]".
>> Can you rule out the writing part by counting the rows? I presume this
>> all happens in a single stage.
>> Enrico
>> Am 18.12.19 um 10:56 schrieb Antoine DUBOIS:
>> Hello
>> I'm working on an ETL based on csv describing file systems to transform
>> it into parquet so I can work on them easily to extract informations.
>> I'm using Mr. Powers framework Daria to do so. I've quiet different input
>> and a lot of transformation and the framework helps organize the code.
>> I have a stand-alone cluster v2.3.2 composed of 4 node with 8 cores and
>> 32GB of memory each.
>> The storage is handle by a CephFS volume mounted on all nodes.
>> First a small description of my algorithm (it's quiet simple):
>> Use SparkContext to load the csv.bz2 file,
>> Chain a lot of withColumn() statement,
>> Drop all unnecessary columns,
>> Write parquet file to CephFS
>> This treatment can take several hours depending on how much lines the CSV
>> is and I wanted to identify if bz2 or network could be an issue
>> so I run the following test (several time with consistent result) :
>> I tried the following scenario with 20 cores and 2 core per task:
>>    - Read the csv.bz2 from CephFS with connection with 1Gb/s for each
>>    node: ~5 minutes.
>>    - Read the csv.bz2 from TMPFS(setup to look like a shared storage
>>    space): ~5 minutes.
>>    - From the 2 previous tests I concluded that uncompressing the file
>>    was part of the bottleneck so I decided to uncompress the file and store it
>>    in TMPFS as well, result: ~5.9 minutes.
>> The test file has 25'833'369 lines and is 370MB compressed and 3700MB
>> uncompressed. Those results have been reproduced several time each.
>> My question here is by what am I bottleneck in this case ?
>> I though that the uncompressed file in RAM would be the fastest. Is it
>> possible that my program is suboptimal reading the CSV ?
>> In the execution logs on the cluster I have 5 to 10 seconds GC time max,
>> and timeline shows mainly CPU time (no shuffling, no randomization overload
>> either).
>> I also noticed that memory storage is never used during the execution. I
>> know from several hours of research that bz2 is the only real compression
>> algorithm usable as an input in spark for parallelization reasons.
>> Do you have any idea of why such a behaviour ?
>> and do you have any idea on how to improve such treatment ?
>> Cheers
>> Antoine
>> --
> Best Regards,
> Ayan Guha

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