Burst VM Sizes - credit drain and throttle occures on half of the defined Base CPU Performance

[Felix_Riedel]

Hi Forum,

it occured to me with the sizes B4ms, B2s, B4s v2 and B2s v2 that Burst credits are already drained on half of the defined Base CPU Performance from the Perspective of the VM Host.

Example: We were informed by the Customer of high CPU Load and Bad Performance of a VM Guest. From inside the VM the Guest reports 90-100 % CPU Load. In Azure Portal we checked Percentage CPU Metric. We see a nearly falt and smooth line around half of the defined Base CPU Performance.

For a "B4s v2" Base CPU Performance is 40 % and the Metric showed a constant line around 20 %.

Checking the "CPU Credit Remaining" Metric we see the VM isn't Banking any Credits. The VM is capped on half the defined Performance.

We switched the VM Size from a the B-size to a D-size, which alleviated the issue, but creates more costs.

Is this a known bug? Is this fixable or is a greater underlying change underway?

Hope someone has an Idea about it.

Best Regards

Felix

[#2]

@Felix_Riedel

Your observed plateaus are consistent with a throttled B4ms (another machine type you indicated you were using), which has a CPU baseline value of 22.5%.  Are you sure you were looking at the right machine type for this example?

[#3]

 @ChrisElkin Thanks for the question. I might've messed up the example a little. So yes, it was a B4ms. 

In the meantime I have found prove that the Base CPU performance was halved, I just can't say exactly when but it was done, but it was  before October 19th, 2023 and its was done for Gen 1 B-Series.

MS learns content is populated from Github, I found the commit where rishabv90 updated the article: Update sizes-b-series-burstable.md · MicrosoftDocs/azure-docs@c348d37 · GitHub