High CPU Steal Time on VMs

Question

Services on certain VMs show latency spikes correlated with CPU steal time. How do you investigate and mitigate?

Accepted Answer

CPU steal time indicates that the hypervisor is unable to provide the requested CPU cycles to the VM, often due to contention on the underlying host. This is a critical performance indicator that needs thorough investigation to ensure application responsiveness and stability. First, I would confirm the CPU steal time using tools like `vmstat -w 1` or `top` on the affected VM, looking specifically at the '%st' column. If readily available, I'd also check the cloud provider's hypervisor-level metrics for the specific host or cluster to validate the VM-level observation and quantify the steal. Next, I'd investigate potential 'noisy neighbors' by examining other VMs running on the same host or within the same resource pool. This could involve looking at the CPU utilization patterns of other instances on that host, though direct visibility might be limited by the provider. If host-level metrics are available, I'd check for overall host CPU saturation or unusual I/O patterns from other workloads. To correlate the latency spikes, I would align the timestamps of the observed latency in the application or service with the periods of high CPU steal time. This would involve using application performance monitoring (APM) tools, distributed tracing, or even application-specific logs to pinpoint when the latency occurred and cross-reference with system-level metrics. Based on the investigation, mitigation strategies would focus on reducing contention. This could involve migrating the affected VM(s) to a different host or availability zone, or if possible, changing the instance family or placement to one with better resource isolation or a lower chance of co-locating with resource-intensive workloads. If migrating or changing instance types isn't immediately feasible, I would explore optimizing the workload on the VM itself. This might involve profiling the application to identify CPU-bound processes, optimizing code, tuning kernel parameters, or offloading specific tasks to other services to reduce the VM's overall CPU demand. Proactive monitoring of hypervisor and VM-level CPU metrics, especially CPU steal time, is crucial for early detection of contention. Implementing alerts for these metrics can help prevent service degradation before it significantly impacts users, allowing for timely intervention and mitigation.

High CPU Steal Time on VMs

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