AI - Performance Methodology in the Cloud – Part 4 – Harshad
15 Questions
0 Views

Choose a study mode

Play Quiz
Study Flashcards
Spaced Repetition
Chat to lesson

Podcast

Play an AI-generated podcast conversation about this lesson

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

    Studying That Suits You

    Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

    Quiz Team

    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.

    More Like This

    CPU Performance-Enhancing Features Quiz
    7 questions
    CPU Cache Memory and Performance
    10 questions
    CPU Performance and Measurement Techniques
    39 questions
    Use Quizgecko on...
    Browser
    Browser