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Questions and Answers
What is performance characterization?
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?
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True or false: The perf tool can be used to identify which part of the code is consuming the greatest number of cycles.
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What is the consequence of false sharing?
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True or false: The top-down hierarchy in performance characterization starts with level 2.
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What tool can be used to profile code and figure out which part of the code is consuming the greatest number of cycles?
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True or false: Flame graphs are used to record data based on CPU cycles spent.
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What is the top-down hierarchy of performance characterization?
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What data does perf record by default?
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What is the purpose of characterization?
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What is the purpose of the ping-pong movement of the cache line?
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What is the purpose of P-states and C-states?
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What tool can be used to record data based on L3 misses and cycle counts?
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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.