How to resolve 20% flatline CPU utilization on Azure Win Server VM

Question

How to resolve 20% flatline CPU utilization on Azure Win Server VM

GNB 20

We have a Win Server 2019 VM on Azure under the B2s general purpose subscription.

Problem: the CPU flatlines at 20% utilization, slowing down server performance to a relative crawl.

It seems as if there is a CPU quota of 20% being enforced. Perhaps a discretionary customer-specified quota was set during the initial provisioning of this VM. How might we resolve this problem?

Additional info:

A B2s subscription (2 vCPUs) is spec'd to have 40% baseline CPU performance. If we had exhausted our credits, we would have expected CPU utilization to flatline at 40%, instead of the 20% that we see.

Accepted answer

3 additional answers

Your answer

Answer 1

TP 125.9K Volunteer Moderator

Hi,

From your description it sounds like the CPU credits are exhausted so the VM is returning to baseline performance for B2s. You may need to switch to larger B series instance or switch to a non-burstable VM instance type. Please see article below for details on burstable VM instances:

https://learn.microsoft.com/en-us/azure/virtual-machines/sizes-b-series-burstable

In the portal you can look at the VM -- Overview -- Monitoring -- see all metrics, and then add credits remaining and credits consumed.

If the above was helpful please click Accept Answer.

Thanks.

-TP

GNB 20 Reputation points

2023-03-17T15:42:46.1166667+00:00

Thank you for that idea; see follow-up below.
GNB 20 Reputation points

2023-03-17T16:00:58.5533333+00:00

This server has the B2s subscription (2 vCPUs), which is spec'd to have 40% baseline CPU performance.

Throughout the week, according to Azure metrics, CPU utilization typically oscillates normally between 10% and 40% (max).

With these facts in mind, does it still make sense that this server sometimes flatlines (pegs) at 20% CPU utilization for several hours or more?

If we had exhausted our credits, we would have expected CPU to flatline at the spec'd baseline of 40%, instead of the 20% we see.

Reference for the 40% baseline CPU performance: https://learn.microsoft.com/en-us/azure/virtual-machines/sizes-b-series-burstable
TP 125.9K Reputation points Volunteer Moderator

2023-03-17T20:16:55.0433333+00:00

For B2s, if credits are available, it can burst to 200 (100 x 2 vCPU). When credits are exhausted, it is limited to baseline of 40, which is 20% of 200.

In your oscillation range, if CPU is at 10%, it would be 20 out of 200, so accruing credits, whereas when the CPU is at 40%, it would be 80 out of 200, so credits would be diminishing. Once credits reach 0 it would be limited to 20% until CPU usage dropped below 20% and allowed credits to accumulate.
GNB 20 Reputation points

2023-03-18T14:55:56.1333333+00:00

Ah. I believe you're saying that for purposes of the Azure 'Percent CPU' metric, it calculates the denominator as the combined burst value of all vCPUs -- for a B2s, 100 x 2 vCPU = 200.

With this same calculation, a B2s is provisioned for 20% CPU (absent credits for occasional bursting).

Thus, it appears the B2s subscription's 'Baseline CPU Performance' provision of 40% is calculated on a different basis than the (corresponding 20%) Azure 'Percent CPU' metric.

Note that the B2s subscription provisions the CPU as 40%, not as 40.

This difference in calculation seems a bit unexpected or unintuitive. Are you sure about this?

Reference for 40% Baseline CPU Performance for a B2s: https://learn.microsoft.com/en-us/azure/virtual-machines/sizes-b-series-burstable
TP 125.9K Reputation points Volunteer Moderator

2023-03-25T18:09:13.9433333+00:00

The Base CPU and Max CPU in the article are for one vCPU.

For example, when you look at the chart for Standard_B1S, it shows 10% for Base column and 100% for Max column. That's 10% and 100% of one vCPU. Likewise if you look at chart for Standard_B16ms it shows 270% and 1600%, which is 270% and 1600% of one vCPU.

It can be confusing the way the article presents it, however, one benefit is you are able to quickly see relative CPU quota differences between the sizes without having to do calculations.

With the above in mind you can see how 40% in the chart for B2s equates to 20% CPU metric since the metric is based on 2 vCPUs and the chart is based on 1 vCPU.
GNB 20 Reputation points

2023-03-25T18:09:38.55+00:00

I believe you're saying that for purposes of the Azure 'Percent CPU' metric, it calculates the denominator as the combined burst value of all vCPUs -- for a B2s, 100 x 2 vCPU = 200.

With this same calculation, a B2s is provisioned for 20% CPU (absent credits for occasional bursting).

Thus, it appears the B2s subscription's 'Baseline CPU Performance' provision of 40% is calculated on a different basis than the (corresponding 20%) Azure 'Percent CPU' metric.

Note that the B2s subscription provisions the CPU as 40%, not as 40.

This difference in calculation seems a bit unexpected or unintuitive. Are you sure about this?

Reference for 40% Baseline CPU Performance for a B2s: https://learn.microsoft.com/en-us/azure/virtual-machines/sizes-b-series-burstable
GNB 20 Reputation points

2023-03-25T18:09:54.89+00:00

After upgrading a subscription (say, from B1s to B2s), I wonder if a Stop / Start of the VM is required at the Azure management layer in order to establish the new baseline CPU performance provision... ?
TP 125.9K Reputation points Volunteer Moderator

2023-03-25T18:13:02.79+00:00

After upgrading a subscription (say, from B1s to B2s), I wonder if a Stop / Start of the VM is required at the Azure management layer in order to establish the new baseline CPU performance provision... ?

When you switch VM size the VM is stopped/started automatically and the new baseline is established.

-TP

Answer 2

vipullag-MSFT 26,492 Moderator

Hello GNB

Welcome to Microsoft Q&A Platform, thanks for posting your query here.

I crossed checked with internal team on this.

For all B series sizes, the spec is ~20% of a given core. For the case of B2s, 40%/2 cores = 20% of a single core at baseline.

Currently, Microsoft just announced a new version of B series VMs (Bv2) – you can try the private preview if interested.

Hope that clarifies.

GNB 20 Reputation points

2023-03-29T15:40:27.16+00:00

Thanks for that. I would indeed like to try a private preview of the new version of the B series subscription (Bv2). Please let me know how.
vipullag-MSFT 26,492 Reputation points Moderator

2023-04-04T02:55:29.89+00:00

Hello GNB Thanks for your response.
I request you to send an email to AzCommunity@Microsoft.com with Subject as "Attn:Vikas" referencing this thread along with your subscription ID.

Answer 3

Deleted

This answer has been deleted due to a violation of our Code of Conduct. The answer was manually reported or identified through automated detection before action was taken. Please refer to our Code of Conduct for more information.

Comments have been turned off. Learn more

Answer 4

GNB 20

..........

Share via

How to resolve 20% flatline CPU utilization on Azure Win Server VM

3 additional answers

Your answer