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Discrepancy in CPU Usage between "Overall CPU Percentage-Usage" and total of App CPU Usage

Joseph Little 0 Reputation points
2025-09-19T14:01:38.3+00:00

Starting around 9/2/2025 we have started to receive timeout on our Web Apps. In response to this we have turned off all the Slots (Staging), restarted our production sites, we have also enable App Insights (9/8/2025) to see if we can see what is occurring. But the issue persists. I see from the High CPU Analysis the "Overall CPU Percent-Usage" far exceeds the sum of all the App CPU usage by a factor of about 5. I.e. Sum of Apps CPU Usage ~ 20% where as Overall CPU usage ~ 100%. Can you help explain this discrepancy? Prior to 9/2/2025 during a CPU spike the Overall CPU usage was roughly twice that of the Sum of the Apps CPU usage, and not 5 times like it is now.
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Thank you for your help.
Joseph

Azure App Service
Azure App Service

Azure App Service is a service used to create and deploy scalable, mission-critical web apps.

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  1. Anonymous
    2025-09-19T16:13:52.03+00:00

    Hi Joseph Little

    Thank you for posting your question on Microsoft Q&A.

    It appears that the App Service Plan may be hosting more active sites than recommended. This can lead to higher overall CPU usage compared to the sum of individual application CPU usage due to kernel services are also included in the overall CPU usage. These can be higher when system processes call APIs frequently, such as I/O, networking, and thread context switching.

    Possible steps to consider:

    Scale Up the App Service Instance Scaling up the App Service instance increases the available CPU, memory, and disk space to handle additional workload.

    Split Applications into Separate App Service Plans To avoid one application impacting the performance of others on the same plan, consider moving production apps into a separate App Service Plan. This ensures that each app receives sufficient dedicated resources.

    As a general guideline, here are the recommended maximum active sites per worker size:

    Worker Size Max Sites
    Small 8
    Medium 16
    Large 32

    If Application Insights Profiler is enabled, you may also try disabling it and observe the impact.

    From the High CPU Analysis, high memory usage has also been detected on multiple instances. This usage is not limited to a single app or process, but rather multiple apps and system processes together (for example: mi-ministry, mi-tasks, mi-admin-api).

    In such scenarios, scaling up the App Service instance or splitting the applications into separate App Service Plans can help improve stability and performance.

    Thanks

    Vimal Lalani

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