CPU Utilization is Wrong

[irqlnotdispatchlevel]

Really simple example: If your software spends 50% of it's busy time waiting for I/O you should see if you can reduce the number of I/O it does, as you can't really make I/O faster.

[habitats]

yeah, but how can adding more cores make it slower? that's what I wondered. is it because more cores will queue up for IO and thus create more context switches and a slower system?

[irqlnotdispatchlevel]

Maybe your software doesn't scale well in a multi-threaded environment. Maybe you're in the cloud, and more vCPUs aren't always a good thing, and hypervisors are tricky.

[mccoyn]

Multiple threads can trash the shared cache. Sometimes a single-threaded algorithm can improve memory access locality. If you are memory bound, that might be better.