NVMe Base 2.4: Power/Thermal Features and Power Management

01 Feature capabilities, reads, and settings

01.01.A Feature is not a simple register. The host first reads capability with SEL=011b, then retrieves current/default/saved views, confirms scope and persistence, and only then writes. Set completion proves command outcome; a follow-up Get and runtime telemetry prove that software observes the new policy.

SEL

Select, the Get Features field choosing current, default, saved, or supported-capabilities view.

01.02.Get Features is the Admin command that retrieves Feature attributes. An engineering flow starts by identifying the FID, querying capability, and retrieving current/default/saved values instead of guessing before a write.

Admin

Administrative, the control path used to create, configure, query, or manage controllers and queues.

FID

Feature Identifier: selects the Feature to read or configure.

NSSPEC

Namespace Specific, the capability bit indicating whether a Feature has per-namespace scope.

CHANG

Changeable, the capability bit indicating whether Set Features can modify the Feature value.

SVBL

Saveable, the supported-capabilities bit indicating whether a Feature can be saved.

02 Power-state power, latency, and performance

02.01.A state number alone cannot establish workload suitability. Read MP, NOPS, ENLAT/EXLAT, IDLP/ACTP, and relative performance together in each PSD. Increasing PS numbers reduce maximum power monotonically, but latency and throughput do not necessarily change by a fixed ratio.

ENLAT

Entry Latency, the maximum latency to enter a power state, in microseconds.

EXLAT

Exit Latency, the maximum latency to exit a power state, in microseconds.

ACTP

Active Power, average active power under the specified workload and time window.

IDLP

Idle Power, typical power under the specification's idle measurement conditions.

NOPS

Non-Operational State, the PSD bit indicating that the state does not process I/O commands.

PSD

Power State Descriptor, the structure describing power, latency, operational type, and relative performance for one power state.

MP

Maximum Power, the sustained maximum power of a power state.

PS

Power State, a controller power/performance operating point; PS0 has the highest maximum power.

02.02.A controller shall support at least one power state and may support up to 32, numbered contiguously from zero. PS0 has the highest maximum power; each subsequent state's maximum power does not exceed the preceding state.

03 APST idle conditions and automatic transitions

03.01.The 256-byte APST buffer is not a performance table. It contains 32 rules stating which non-operational state to enter after a given idle duration. APSTE enables timer rules, entries with ITPT=0 are inactive, and arriving I/O returns the controller to its most recent operational state.

APSTE

Autonomous Power State Transition Enable, the bit enabling APST-table timer evaluation.

ITPT

Idle Time Prior to Transition, the APST-entry idle threshold in milliseconds.

ITPS

Idle Transition Power State, the target non-operational power state selected by an APST entry.

03.02.FID 0Ch APSTE=1 enables Autonomous Power State Transition (APST); the default is zero. Enabling it allows controller transitions based on APST-table idle timers; it does not guarantee entry into a particular state.

NOPPME

Non-Operational Power State Permissive Mode Enable, controlling whether controller background work may temporarily exceed a non-operational power limit.

04 Temperature thresholds, sensors, and notifications

04.01.FID 04h is more than a temperature number. TMPSEL selects a sensor, THSEL selects over or under, TMPTH sets the trigger point, TMPTHH sets the event clear point, and SMART/Health.TTC plus AEC enable return controller state to the host.

TMPSEL

Temperature Sensor Select, the field choosing Composite Temperature or sensor 1 through 8.

TMPTHH

Temperature Threshold Hysteresis, Kelvin hysteresis used when ending a threshold event.

THSEL

Threshold Type Select, choosing an over-temperature or under-temperature threshold.

TMPTH

Temperature Threshold, a 16-bit threshold value in Kelvin.

TTC

Temperature Threshold Critical Warning, the temperature-threshold bit in SMART/Health Critical Warning.

04.02.FID 04h sets over/under thresholds for Composite Temperature and up to eight implemented temperature sensors. Temperature is encoded in Kelvin; reaching an over threshold or falling to/below an under threshold may set the SMART/Health Temperature Threshold critical warning and trigger an asynchronous event.

05 Two levels of HCTM thermal management

05.01.HCTM does not select a fixed clock or power state. It gives the controller two temperature boundaries, TMT1/TMT2. At TMT1 the controller minimizes performance impact; at TMT2 it applies stronger thermal control. Actual hysteresis and internal actions are vendor implementation details.

TMT1

Thermal Management Temperature 1, the lighter thermal-management threshold in Kelvin.

TMT2

Thermal Management Temperature 2, the stronger thermal-management threshold in Kelvin.

05.02.FID 10h uses TMT1[31:16] as the lighter thermal-management threshold and TMT2[15:0] as the heavier threshold, both in Kelvin; zero independently disables the corresponding threshold.

MNTMT

Minimum Thermal Management Temperature, the minimum Kelvin value accepted for HCTM.

MXTMT

Maximum Thermal Management Temperature, the maximum Kelvin value accepted for HCTM.