perf stat --smi-cost estimates what percentage of CPU cycle time was spent handling System Management Interrupts (SMIs) during a measurement interval. It is useful for finding aggregate SMI overhead, but it is not a worst-case latency test: many short SMIs can hide a rare, much longer interruption.
What perf stat --smi-cost measures
SMIs are handled in system-management mode and temporarily take control away from the operating system. The perf SMI-cost metric estimates the share of cycles attributed to that handling over the interval you measure.
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The documented calculation is:
(APERF - unhalted core cycles) / APERF
Perf presents the result as a percentage by default. During collection it enables the CPU’s freeze-on-SMI behavior through /sys/device/cpu/freeze_on_smi. Core counters therefore stop advancing while an SMI is active, whereas APERF remains unaffected; the difference is used to estimate SMI-affected cycles.
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Run the measurement
Show the percentage metric
perf stat --smi-costThis is the concise, metric-only view. Without a command, perf measures the relevant interval until you stop it, normally with Ctrl-C.
Expose the underlying counters
perf stat --smi-cost --no-metric-only--no-metric-only asks perf to print the underlying values as well as (or instead of only) the derived metric. Those values let you relate estimated SMI cycles to the reported SMI count and estimate an average cycles-per-SMI figure for the interval.
Measure a specific workload
perf stat --smi-cost --no-metric-only <command>Replace <command> with the program or benchmark you want to observe. Keep the workload, duration, CPU placement, and background activity consistent when comparing systems or software versions.
Reading the output
The percentage
A result such as a small fraction of a percent means that, in aggregate, only that share of measured cycles was attributed to SMI handling. It does not mean every SMI was short, nor does it establish that no latency-sensitive interruption occurred.
The raw values and average duration
The non-metric view includes values from which you can estimate average SMI cost by relating estimated SMI cycles to the SMI count. An average is only a summary: a population of many short events plus one long event can produce the same average as a population of uniformly moderate events.
Idle intervals
If the machine is mostly idle and no SMIs occur, counter relationships can look inconsistent with the metric’s documentation. Treat an idle capture as a special case rather than as proof that the platform has no SMI problem. Repeat the measurement during the workload that matters.
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What the option requires
- Perf must support the
msr/aperf/andmsr/smi/events on the target machine. - The CPU, firmware, kernel, and perf build must expose the required model-specific registers (MSRs).
- The measurement needs the kernel’s freeze-on-SMI support used by this metric.
The feature is documented as available from Linux kernel 4.13 and on Intel x86 processors dating from roughly 2008 onward when the necessary MSRs exist. Those are historical compatibility guidelines, not a guarantee for a particular system. Check the actual CPU, running kernel, perf version, and event availability before basing a diagnosis on the result.
Why perf may report that SMI cost is unsupported
An unsupported message generally means that one or more required events or controls are unavailable, rather than that the command syntax is wrong. Check the following:
- Confirm that the running kernel and the installed perf tool are from a compatible release.
- Verify that the platform exposes both
msr/aperf/andmsr/smi/events; a CPU family that lacks the relevant MSRs cannot provide this metric. - Check that the kernel can use freeze-on-SMI for the CPU counters.
- Retry on the actual target kernel and hardware, not only in a virtual machine or on a different host; virtualized environments may not expose these MSRs.
If any prerequisite is missing, do not substitute a percentage from another counter and label it SMI cost. Use a platform that exposes the required events or treat SMI overhead as unmeasured.
Using the result in a latency investigation
For aggregate performance impact
Run the percentage view over a representative workload. A consistently non-zero share indicates that SMIs consumed measurable cycle time during that interval. Compare captures only when hardware, kernel and perf versions, workload, and measurement duration are held as constant as practical.
For average SMI cost
Use --no-metric-only and relate the estimated SMI cycles to the SMI count. The resulting average can indicate that handling is taking longer than expected for that workload, but the sources do not establish a universal acceptable percentage or duration. A real-time or tightly latency-bound system may require investigation at values that are harmless for a batch workload.
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Do not treat a low SMI percentage or low average as a latency guarantee. Rare long SMIs are diluted by numerous short ones. If deadline misses or jitter matter, preserve the raw counts, repeat captures under the triggering workload, and investigate outliers with a method capable of showing event-duration distribution; --smi-cost alone supplies an aggregate estimate, not that distribution.
Quick Recap
Practical measurement checklist
- Record CPU model, firmware configuration, kernel release, and perf version.
- Confirm support for
msr/aperf/,msr/smi/, and freeze-on-SMI before interpreting numbers. - Choose a representative, finite workload and state the measurement interval.
- Use the default command for a quick percentage; add
--no-metric-onlywhen you need counts and estimated average cost. - Repeat runs and compare like with like; do not compare an idle capture with a loaded capture.
- Report the percentage as aggregate cycle share and label any average as an average, never as a worst-case bound.
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