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  1. ORC
  2. ORC-1136

Optimize reads by combining multiple reads without significant separation into a single read

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Details

    • Improvement
    • Status: Closed
    • Major
    • Resolution: Fixed
    • 1.7.3
    • 1.8.0
    • Java
    • None

    Description

      Background

      We are moving our workloads from HDFS to AWS S3. As part of this activity we wanted to understand the performance
      characteristics and costs of using S3.

      Seek vs Read

      One particular scenario that stood out in our performance testing was Seek vs Read when dealing with S3.

      In this test we are trying to read through a file

      • Seek to Point A in the file read X bytes
      • Move to Point B in the file that is A + X + Y
      • This is accomplished as another seek or as a read
      • We will leave Y variable to determine when this is best
      • Read X bytes

      Observations:

      • We could clearly see that a read is more performant than seek when dealing with steps/gaps smaller than 4 MB.
        • At 4 MB read is faster by ~ 11%
        • At 1 MB read is faster by ~ 20%
      • Reads are also cheaper as we perform a single GET instead of multiple GETs from AWS S3 Pricing
        • Cost for GET: $0.0004
        • Cost for Data Retrieval to the same region AWS EKS: $0.0000

      ORC Read

      Based on the above performance penalty when dealing with multiple seeks over small gaps, we measured the performance of
      ORC read on a file.

      File details:

      • Size: ~ 21 MB
      • Column Count: ~ 400
      • Row Count: ~ 65K

      Observations:

      • We can clearly see that we pay a significant penalty when reading alternate columns, which in the current implementation of ORC translates to multiple GET calls on AWS S3
      • While the impact of penalty will be less significant in large reads, it will incur overheads both in terms of time and cost

      Read Optimization

      The following optimizations are proposed:

      • orc.min.disk.seek.size is a value in bytes: When trying to determine a single read, if the gap between two reads
        is smaller than this then it is combined into a single read.
      • orc.min.disk.seek.size.tolerance is a fractional input: If the extra bytes read is greater than this fraction of
        the required bytes, then we drop the extra bytes from memory.
      • We can further consider adding an optimization for the complete stripe in case the stripe size is smaller than
        `orc.min.disk.seek.size`

      Scope

      Different types of IO takes place in ORC today.

      • Reading of File Footer: Unchanged
      • Reading of Stripe Footer: Unchanged
      • Reading of Stripe Index information: Optimized
      • Reading of Stripe Data: Optimized

      Each of the above happens at different stages of the read. The current implementation optimizes reads that happen using the DataReader interface.

      This does not:

      • Optimize the read of the file/stripe footer
      • Reads across multiple stripes

      Benchmarks

      In this benchmark we brought up an EKS Container in the same region as the AWS S3 bucket to test the performance of the patch.

      Observations/Details:

      • Input File details:
        • Rows: 65536
        • Columns: 128
        • FileSize: ~ 72 MB
      • Full Read (alternate = false)
        • No significant difference between the options as expected
      • Alternate Read (alternate = true)
        • We get a significant boost in performance 5.8s without optimization to 1.5s with optimization giving us a time
          reduction of ~ 75 %
        • This also gives us a cost saving as 64 GET one for each column per stripe have been replaced with a single GET
        • We can see a marginal improvement ~ 3% when choosing to retain extra bytes (extraByteTolerance=10.0) as compared to
          (extraByteTolerance=0.0) which performs additional work of dropping the extra bytes from memory.

      Summary

      Based on the benchmarks the following is recommended for ORC in AWS S3:

      • `orc.min.disk.seek.size` is set to `4194304` (4 MB)
      • `orc.min.disk.seek.size.tolerance` is set to value that is acceptable based on the memory usage constraints. When set
        to `0.0` it will always do the extra work of dropping the extra bytes.

      Attachments

        1. seekvsread.png
          32 kB
          Pavan Lanka
        2. orc_read.png
          25 kB
          Pavan Lanka
        3. benchmark.png
          141 kB
          Pavan Lanka

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            planka Pavan Lanka
            planka Pavan Lanka
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