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From Wenchen Fan <cloud0...@gmail.com>
Subject Re: Fw:Re:Re: A question about radd bytes size
Date Tue, 03 Dec 2019 04:30:30 GMT
You can only know the actual data size of your RDD in memory if you
serialize your data objects to binaries. Otherwise you can only estimate
the size of the data objects in JVM.

On Tue, Dec 3, 2019 at 5:58 AM zhangliyun <kellyzly@126.com> wrote:

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> -------- 转发邮件信息 --------
> 发件人:"zhangliyun" <kellyzly@126.com>
> 发送日期:2019-12-03 05:56:55
> 收件人:"Wenchen Fan" <cloud0fan@gmail.com>
> 主题:Re:Re: A question about radd bytes size
>
> Hi Fan:
>    thanks for reply,  I agree that the how the data is stored decides the
> total bytes of the table file.
> In my experiment,  I found that
> sequence file with gzip compress is 0.5x of the total byte size calculated
> in memory.
> parquet file with lzo compress is 0.2x of the total byte size calculated
> in memory.
>
> Here the reason why  actual hive table size is  less than total size
> calculated in memory is decided by format sequence, orc, parquet and others.
> Or is decided by compress algorithm Or both?
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> Meanwhile can I directly use org.apache.spark.util.SizeEstimator.estimate(RDD)
> to estimate the total size of a rdd? I guess there is some difference
> between the actual size and estimated size. So in which case, we can use or
> in which case we can not use.
>
> Best Regards
> Kelly Zhang
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> 在 2019-12-02 15:54:19,"Wenchen Fan" <cloud0fan@gmail.com> 写道:
>
> When we talk about bytes size, we need to specify how the data is stored.
> For example, if we cache the dataframe, then the bytes size is the number
> of bytes of the binary format of the table cache. If we write to hive
> tables, then the bytes size is the total size of the data files of the
> table.
>
> On Mon, Dec 2, 2019 at 1:06 PM zhangliyun <kellyzly@126.com> wrote:
>
>> Hi:
>>
>>  I want to get the total bytes of a DataFrame by following function , but
>> when I insert the DataFrame into hive , I found the value of the function
>> is different from spark.sql.statistics.totalSize .  The
>> spark.sql.statistics.totalSize  is less than the result of following
>> function getRDDBytes .
>>
>>    def getRDDBytes(df:DataFrame):Long={
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>>   df.rdd.getNumPartitions match {
>>     case 0 =>
>>       0
>>     case numPartitions =>
>>       val rddOfDataframe = df.rdd.map(_.toString().getBytes("UTF-8").length.toLong)
>>       val size = if (rddOfDataframe.isEmpty()) {
>>         0
>>       } else {
>>         rddOfDataframe.reduce(_ + _)
>>       }
>>
>>       size
>>   }
>> }
>> Appreciate if you can provide your suggestion.
>>
>> Best Regards
>> Kelly Zhang
>>
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>>
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