Azure function app scaled out but the performance of each instance are not equal

Question

I have a durable function app to handle xml file in blob which size is between a few megabytes and hundreds of megabytes.

The requirement requires up to 20 files to be process at the same time. I've scaled out the durable function app to 4 instances, but when requests increase rapidly, only 2 instances encountered the problem of too high CPU, while the other 2 did not.

This results in very slow file processing.

Is there a problem with azure's built-in load balance?

See this picture to check the high CPU issue

Answer 1

• Generally, the scaling logic in Azure Functions currently works well when the function is triggered by things like queues or Event Hubs.
• if you're running your Function in an App Service (also known as a Dedicated Plan), it will by default only scale instances within the possibilities of the App Service Plan you defined.

Using an App Service plan, you can manually scale out by adding more VM instances. You can also enable autoscale, though autoscale will be slower than the elastic scale of the Premium plan. [...] You can also scale up by choosing a different App Service plan.

• If you run your Function App on Consumption Plan (the true serverless hosting plan option since it enables scaling to zero)
• For further information check the below provided links.

1. Azure Functions Scalability Issue.
2. Azure Functions Hosting Plan.
3. Consumption Plan Scaling Issues.