How Can Identical Servers Have Different CPU Utilization Under The Same Load?

I ran into an interesting case today. We’ve got a pair of SQL Servers running in an Availability Group. We’re set up so we can run with either server as the primary. We use Sentry One to monitor our SQL servers and after a recent planned failover, I noticed that when we run on Server 1, CPU processor utilization seems to be lower than when we are running on Server 2. Not drastically lower and there is not enough difference that our users would complain about it, but when I looked at the graphs, I could definitely see a difference.

This bothered me. The two servers have identical hardware and SQL Server is configured identically on both. I ran through all the checks I could think of: power management was set to High Performance on both, NIC drivers were the same version, both were using the same SAN and local drives for storage, memory was the same, anti-virus settings were identical. I just couldn’t explain why Server 1 was always running with about a 10% lower CPU utilization.

Server 1 on the left, Server 2 on the right

Normally, I like seeing a low CPU utilization. I like my SQL Servers to have some headroom to handle an increase in load. We had been running on Server 1 for a long time in the past and, looking at the CPU utilization, I thought we had lots of room to handle an increased load. But when we were running on Server 2, I wasn’t so sure.

One morning, I decided to dig a little deeper. I had already confirmed Windows power management was set to High Performance on both servers. However, this time, I dug a little deeper and checked out what those settings actually were. Turns out, that was where the difference was.

I discovered that Server 2 was set to a maximum processor state of 100%, but Server 1 was set to 75%. Aha! That was the reason Server 1 always had a lower CPU utilization! Windows was limiting processor use to 75%!

Digging deeper still, I found several other settings that were different: minimum processor state, hard drive shutoff time, PCI Express link state power management, and one or two others.

The moral is just because your CPU utilization looks low, make sure that it isn’t because there is some other rule limiting your processor utilization!

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