AI - Performance Methodology in the Cloud – Part 4 – Harshad
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AI - Performance Methodology in the Cloud – Part 4 – Harshad

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Questions and Answers

What is performance characterization?

  • A process where the performance monitoring unit (PMU) within the CPU allows you to collect certain counters
  • A process of determining the cause of a performance issue (correct)
  • A process of analyzing the code fragments that are caused by performance issues
  • A process of profiling code and figuring out which part of the code is consuming the greatest number of cycles
  • True or false: False sharing is where two independently declared variables are accessed by the same thread on a processor.

    False

    True or false: P-states and C-states have no effect on performance.

    False

    What is false sharing?

    <p>When two independent declared variables that are independently accessed by different threads on a processor lie on the same cache line</p> Signup and view all the answers

    True or false: The perf tool can be used to identify which part of the code is consuming the greatest number of cycles.

    <p>True</p> Signup and view all the answers

    What is the consequence of false sharing?

    <p>Diminished performance</p> Signup and view all the answers

    True or false: The top-down hierarchy in performance characterization starts with level 2.

    <p>False</p> Signup and view all the answers

    What tool can be used to profile code and figure out which part of the code is consuming the greatest number of cycles?

    <p>perf</p> Signup and view all the answers

    True or false: Flame graphs are used to record data based on CPU cycles spent.

    <p>False</p> Signup and view all the answers

    What is the top-down hierarchy of performance characterization?

    <p>Frontend bound, latency bound, and L3 misses</p> Signup and view all the answers

    What data does perf record by default?

    <p>CPU cycles spent</p> Signup and view all the answers

    What is the purpose of characterization?

    <p>To determine the cause of a performance issue</p> Signup and view all the answers

    What is the purpose of the ping-pong movement of the cache line?

    <p>To maintain the consistency and coherency of the caches</p> Signup and view all the answers

    What is the purpose of P-states and C-states?

    <p>To improve the performance of the CPU when it doesn't have intermittent work to do</p> Signup and view all the answers

    What tool can be used to record data based on L3 misses and cycle counts?

    <p>perf</p> Signup and view all the answers

    Study Notes

    • Performance characterization is a process where the performance monitoring unit (PMU) within the CPU allows you to collect certain counters, and some of these counters can identify patterns.
    • A pattern here is false sharing. False sharing is where two independent declared variables that are independently accessed by different threads on a processor lie on the same cache line, which is a unit of access for a processor within the cache.
    • Even though from a software point of view, thread-0 is accessing variable-A and thread-1 is accessing variable-B, because these are on the same hardware cache line, the moment thread-0 makes changes to this cache line, it has to ping-pong the cache line to the other CPU, to make sure the change is consistent.
    • If thread-1 makes a change, it must go back, make sure it's consistent, and then ping-pong back. This ping-pong movement of the cache line that’s needed to maintain the consistency and coherency of the caches, ends up giving a diminished performance result.
    • Through characterization at runtime, you can find out that the P-states and the C-states, meaning the power and idle states of the CPU, have a lot to do with this problem. No software can easily detect that because a CPU goes to sleep when it doesn’t have intermittent work to do.
    • You can see that, based upon C-state levels that are set to default, which is 9 on the cloud, there's a latency excursion and then it goes down again.
    • Characterization is the process of determining the cause of a performance issue.
    • Characterization can be done with the help of tools such as perf or PerfSpect.
    • The top-down hierarchy starts at level 1, where the CPU can be stalled because it's frontend bound, meaning it's not getting any instructions to execute.
    • Under each of these levels, you have other levels that tell you where you were bound in the frontend.
    • If it was latency, it tells you where you were latency bound.
    • To figure out which part of the service is blocked, imagine that after the map and zoom, you're left with the block diagram shown below.
    • You can then scrutinize the code fragments that are caused by these issues.
    • perf is a tool that can be used to profile code and figure out which part of the code is consuming the greatest number of cycles
    • perf records data based on L3 misses and cycle counts
    • by default, most perf records are based on CPU cycles spent, but by using flame graphs, we can also record data based on L3 misses

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    Description

    Explore the process of performance characterization and profiling within the CPU, including the identification of false sharing patterns, the impact of P-states and C-states, and the use of tools like 'perf' for code profiling. Learn how to pinpoint performance issues and optimize code for improved efficiency.

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