Understand of CPU state

The different values for the max_cstate parameter in Linux kernel power management settings refer to the maximum allowed CPU idle state, or C-state, that the system can use to save power. The different C-states represent different levels of power management and performance trade-offs.

Here's a brief overview of the different C-states:

C0: This is the active state, where the CPU is fully powered and executing instructions.

C1: This is the first idle state, where the CPU stops executing instructions and enters a low-power state. The CPU can still respond quickly to interrupts and events, but power consumption is reduced.

C2: This is a deeper idle state, where the CPU stops executing instructions and enters a lower-power state than C1. The CPU takes longer to respond to interrupts and events, but power consumption is further reduced.

C3: This is a deeper idle state than C2, where the CPU stops executing instructions and enters an even lower-power state. The CPU takes even longer to respond to interrupts and events, but power consumption is further reduced.

The max_cstate parameter in the Linux kernel power management settings controls the maximum allowed C-state, or the deepest idle state that the system can use to save power. The value of the max_cstate parameter represents the maximum allowed C-state, where 0 represents the active state (C0) and higher values represent deeper idle states.

Setting max_cstate to 0 disables CPU power management features and keeps the CPU in the active state (C0) at all times. This maximizes performance and minimizes latency, but also increases power consumption and heat generation.

Setting max_cstate to 1 enables CPU power management features and allows the CPU to enter the first idle state (C1) to save power when it's not in use. This balances performance and power management, but may still result in higher power consumption and heat generation than deeper idle states.

Setting max_cstate to 3 enables deeper CPU power management features and allows the CPU to enter deeper idle states (C2 and C3) to save more power when it's not in use. This maximizes power management, but may also result in longer response times and potential performance penalties when the CPU wakes up from deeper idle states.

The choice of max_cstate value depends on the specific hardware, workload, and power management requirements of the system. In general, deeper idle states result in more power savings but longer response times, while shallower idle states result in less power savings but faster response times.