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  1. MariaDB ColumnStore
  2. MCOL-4849

Optimize ExeMgr to reduce a number of context switches

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    Details

    • Type: New Feature
    • Status: Closed (View Workflow)
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: 6.1.1
    • Fix Version/s: 6.2.2
    • Component/s: ExeMgr
    • Labels:
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      Description

      MCS generates enourmous amount of context switches that degrades performance a lot. The screenshoot cs.png demonstates this, namely cs number raises from ~80 to 21k once I run a single query(select l_orderkey, count(l_orderkey) from lineitem group by l_orderkey limit 10) in an infinite loop.
      According with the perf tool observations(collected with perf record --call-graph dwarf -e context-switches -p $(pidof ExeMgr) command) there are number of candidates for the optimization:

      • messageqcpp::InetStreamSocket::readToMagic() that both uses poll and reads a byte at a time increasing the number of cs needed.
      • joblist::TupleBPS::receiveMultiPrimitiveMessages() that contains a loop that processes intermediate RGData sent by PP to EM as results of Primitive requests. There are both mutex and conditional_variables widely used in TupleBPS methods's code.
      • joblist::FIFO<rowgroup::RGData>::swapBuffers() that leverages mutexes to save crit section of this RGData queue from TBPS to TAS.
        Plz take a look at perf.png for some details.
        The goal is to reduce a number of context switches produced by ExeMgr code. It is worth to have in mind that othere categories of queries might produce a different pattern. However the three parts mentioned affect every query b/c the are substantial to query processing.

        Attachments

        1. cs.png
          cs.png
          226 kB
        2. graph.svg
          23 kB
        3. perf.png
          perf.png
          485 kB
        4. profile.txt
          109 kB
        5. Screenshot from 2021-09-01 20-22-08.png
          Screenshot from 2021-09-01 20-22-08.png
          119 kB

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              Assignee:
              dleeyh Daniel Lee
              Reporter:
              drrtuy Roman
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                Updated:
                Resolved:

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