§2024-12-03

Real, User and Sys process time statistics

One of these things is not like the other. Real refers to actual elapsed time; User and Sys refer to CPU time used only by the process.

Real is wall clock time - time from start to finish of the call. This is all elapsed time including time slices used by other processes and time the process spends blocked (for example if it is waiting for I/O to complete).

User is the amount of CPU time spent in user-mode code (outside the kernel) within the process. This is only actual CPU time used in executing the process. Other processes and time the process spends blocked do not count towards this figure.

Sys is the amount of CPU time spent in the kernel within the process. This means executing CPU time spent in system calls within the kernel, as opposed to library code, which is still running in user-space. Like 'user', this is only CPU time used by the process. See below for a brief description of kernel mode (also known as 'supervisor' mode) and the system call mechanism.

User+Sys will tell you how much actual CPU time your process used. Note that this is across all CPUs, so if the process has multiple threads (and this process is running on a computer with more than one processor) it could potentially exceed the wall clock time reported by Real (which usually occurs). Note that in the output these figures include the User and Sys time of all child processes (and their descendants) as well when they could have been collected, e.g. by wait(2) or waitpid(2), although the underlying system calls return the statistics for the process and its children separately.

　　　 Real 420m12.897s user 2348m10.470s sys 86m56.229s

To compare the two machines and determine which one performed better for the same task, let's analyze the three time metrics:

Real time (Wall-clock time):

orgpi516G: 780m7.331s (about 780 minutes or 13 hours) second machine: 420m12.897s (about 420 minutes or 7 hours) Analysis: The second machine finished the task in 420 minutes, while the first machine took 780 minutes. The second machine was much faster, completing the task in about half the time.

User time (CPU time spent on the task by your program):

orgpi516G: 1818m56.729s (about 1819 minutes or 30.3 hours) second machine: 2348m10.470s (about 2348 minutes or 39.1 hours) Analysis: The second machine spent more user time (39.1 hours vs 30.3 hours), which suggests that while it finished the task faster in wall-clock time, it might have been doing more computation or used resources differently (e.g., maybe more parallelization or higher efficiency in the other machine).

System time (CPU time spent on kernel operations like I/O):

orgpi516G: 100m38.168s (about 100 minutes) second machine: 86m56.229s (about 87 minutes) Analysis: The system time is relatively close between the two machines, with the first machine spending a little more time on system operations.

Conclusion:

The second machine is clearly the better performer in terms of real (wall-clock) time, completing the task in half the time. However, the second machine spent more user time, meaning that its CPU was more heavily utilized for the task, which might imply a difference in task parallelism, load, or resource management. Given that real time is usually the most important metric in practical terms (unless you have a specific reason to analyze user/system time), the second machine appears to be the better choice for this task, as it completes the job faster in real time.

あなた: comapre with User + Sys time is more accurate ChatGPT 4o mini

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サインアップ あなた: comapre with User + Sys time is more accurate　in comparing machines??? ChatGPT: ChatGPT Yes, comparing User + Sys (system) time can be a more accurate and holistic way of comparing the performance of different machines, especially when you're benchmarking programs that make use of both user-level code and kernel-level (system) code.

Here’s why:

User Time:

This is the amount of time the CPU spends executing your program’s code (user-space). It's mostly impacted by the algorithms and data structures you're using in your program. It directly reflects how well your program's logic is optimized. Sys Time:

This is the amount of time the CPU spends executing kernel-level operations on behalf of your program (e.g., system calls, I/O operations, memory management). It reflects the efficiency of the underlying operating system and hardware in handling system calls and resource management. High sys time might indicate that your program is frequently interacting with the OS, and optimizations in these areas could potentially improve performance. Why combining User + Sys time is useful: Holistic View: If you only look at user time, you might miss how much time is spent interacting with the system or how efficient the OS is at handling system calls. Likewise, looking only at sys time might mask inefficiencies in your actual program logic.

System Bottlenecks: High sys time could indicate a bottleneck in resource access, system calls, or the OS scheduler, which could be affecting overall performance. It's important to track both user and sys time to understand the total CPU time consumed.

Machine Comparisons: When comparing two machines, a direct comparison of User + Sys time can give a clearer picture of which machine is more efficient overall, as it accounts for both user-level processing and system-level handling. Differences in hardware (e.g., CPU architecture, I/O speeds) and software (e.g., OS optimizations) can both affect these times.

Example Scenario: Imagine you’re running a program that reads and processes a large file. The User time will tell you how fast your program is at processing that data, but the Sys time will include the time spent on I/O operations (like reading from disk). A machine with better disk performance might have a lower sys time, but a machine with a more powerful CPU could have better user time. Together, they give you a better idea of total time spent.

Caveat: If your program is mostly CPU-bound, User time might be the most important metric. If your program is I/O-bound, Sys time could be more informative. For example, a program that spends a lot of time interacting with a network or disk might show significant system time. Conclusion: For comparing machines in terms of overall efficiency and understanding both application and system-level behavior, looking at User + Sys time is generally more accurate and insightful than focusing on just one of these metrics. It gives a clearer picture of how well a machine handles both the user-level computation and system-level operations.

ChatGPT の回答は必ずしも正しいとは限りません。重要な情報は確認するようにしてください。 ?

ChatGPT の回答は必ずしも正しいとは限りません。重要な情報は確認するようにしてください。 ?