Gina Johnson
The question of whether or not sleep uses CPU time is an important one, especially when considering the various processes that spend most of their lives in a sleep status, such as daemons, servers, and listeners. While it may seem counterintuitive, the CPU is generally not active during sleep mode, and in some cases, it is even powered off. This is true for both traditional machines and newer systems with modern standby modes. However, it's worth noting that the operating system (OS) plays a crucial role in managing the sleep state, and it can still suspend execution and schedule threads to wake up at specific times or in response to certain events. The specifics of the tasks and the system configuration will ultimately determine the CPU utilisation during sleep.
| Characteristics | Values |
|---|---|
| Do sleeping processes consume CPU resources? | No, they do not. |
| Do sleeping processes get the same CPU time? | No, they do not. |
| Does the CPU need to be checking every cycle whether x seconds have passed? | Yes, but the number of CPU cycles allocated to the program is reduced. |
| Does the CPU execute any code in sleep state S2 or greater? | No, it does not. |
| Does the CPU execute any code in the S0ix state? | No, but the system as a whole is still active enough to remain connected to the network and respond to any events. |
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What You'll Learn
- The CPU is powered off in sleep state S2 or greater
- The OS can suspend execution thousands of times a second
- The CPU does not execute any code in sleep state S0ix
- Linux's CFS attempts to give programs increased CPU time in proportion to sleep time
- Sleep is not processor-intensive and allows the processor to idle
The CPU is powered off in sleep state S2 or greater
Sleep states are designated S1, S2, S3, and S4, with S5 being the shutdown state. A system in one of these sleep states is not performing any computational tasks and appears to be off. The CPU is powered off in sleep state S2 or greater.
Sleep state S1 is a low wake-latency sleeping state where the entire system context is preserved with the exception of CPU caches. In this state, the hardware retains all state, and wake latency is very low, in the order of a few microseconds. Typically, this state is where CPU and major hardware blocks (caches, memory fabric, and so on) are clock-gated and clock sources such as PLLs (phase-locked loops) are shut off.
Sleep state S2 is also a low wake-latency sleep state, similar to S1, except that the CPU and system cache context are lost. In this state, the cache and CPU may enter a power-gated state. After a wake event, control starts from the processor's reset vector.
Sleep state S3 is commonly referred to as standby, sleep, or suspend to RAM. It is also a low wake-latency sleep state, but in this state, all CPU, cache, and hardware system context is lost, and only system memory (DRAM) state is maintained.
Sleep state S4 is the lowest-powered sleeping state and has the longest wake-up latency. To reduce power consumption to a minimum, the hardware powers off all devices, although the operating system context is maintained in a hibernate file.
In summary, the CPU is powered off in sleep state S2 or greater, with progressively more of the system being shut down in each successive sleep state.
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The OS can suspend execution thousands of times a second
The operating system (OS) is responsible for scheduling threads and can place a thread in a sleep state that can be woken up by a timer. This is done to ensure that other processes can run. The OS can suspend execution thousands of times a second.
When a process is in a sleep state, it is not executing on the CPU and therefore uses zero CPU time. However, it may add some overhead since the kernel has to manage these processes. The OS will send a halt instruction to the CPU when every single thread is sleeping, telling it that there is nothing else for it to do at that moment. The CPU will then handle the actual sleeping when there are more cycles than running programs.
A process can be in different states, such as the running state, blocked or wait state, terminated or completed state, and the ready state. A process in the running state is currently being executed by the CPU. If there is only one CPU, only one process can be in this state at a time. The blocked or wait state means the process cannot continue executing and is waiting for an event to happen, such as the completion of an input/output operation. In the terminated or completed state, the process has finished its execution or has been stopped by the user. It is then released by the OS and removed from memory.
The ready state means that the process is ready to run and is waiting to get CPU time for its execution. The OS chooses a process from the ready queue for execution. The process then moves to the running state. A suspended process is one that is turned off and does not get scheduled for execution. It can be in this state due to a lack of main memory or because it has been suspended by the user or system. Once the suspension is removed, the process will start execution.
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The CPU does not execute any code in sleep state S0ix
Sleep states are denoted by S0 to S5, with higher S-numbers indicating deeper levels of sleep. S0 is an operational system, while S1, S2, S3, and S4 are various levels of idle that are transparent to the end user. In sleep state S2 or greater, the CPU is powered off.
Intel introduced a new S0ix active idle state, promising the same power consumption as S3 sleep, but with a quick enough wake-up time to return to S0. This is considered an active state, and the SoC as a whole is still active enough to remain connected to the network and respond to events.
However, in the S0ix state, the CPU does not execute any code. This is because, in this state, the CPU is in its deepest runtime idle platform state, also known as DRIPS. This is a non-standard state specific to Intel (x86) SoCs. While the CPU does not execute any code in this state, the system as a whole is still active and can respond to events.
The S0ix state is an example of Intel's focus on understanding the latency tolerance of all components in the system. This allows for better alignment of hardware and OS platform power management, so that the system wakes up for a short period to do work and then quickly returns to sleep.
In summary, while the S0ix sleep state keeps the system active at a minimal level, the CPU itself does not execute any code and remains in its deepest idle state.
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Linux's CFS attempts to give programs increased CPU time in proportion to sleep time
Linux's CFS (Completely Fair Scheduler) attempts to allocate increased CPU time to programs in proportion to their sleep time. This means that a process that sleeps a lot will receive a higher priority when it is resumed, resulting in more CPU time. Essentially, CFS aims to provide a fair distribution of CPU resources by prioritizing processes that have been inactive.
This mechanism is particularly relevant for processes that spend a significant amount of their "lives" in a sleep state. Examples of such processes include daemons, servers, and general listeners. By allocating more CPU time to these sleeping processes, CFS ensures that they are not neglected or starved of resources.
The CFS scheduler in the Linux Kernel is designed to model an "ideal, precise multitasking CPU" on real hardware. It aims to achieve perfect parallelism by running each task at an equal speed of 1/nr_running, where nr_running is the number of tasks running concurrently. In practice, as real hardware can only execute one task at a time, CFS introduces the concept of "virtual runtime" to manage the scheduling of tasks.
The virtual runtime of a task represents when its next timeslice would start execution on the ideal multitasking CPU. In CFS, the virtual runtime is tracked using the per-task p->se.vruntime value, which is updated as tasks utilize the CPU. CFS then selects the task with the smallest p->se.vruntime value to execute, ensuring that tasks are executed in a fair order.
While CFS strives for fairness, there have been observations of increased CPU usage when using CFS compared to other scheduling algorithms like BFS (Brain Fuck Scheduler). Some users have noted higher CPU usage in processes that should be sleeping, indicating potential interactions between the sleeping mechanisms and the scheduler. However, the majority of CFS's design revolves around efficiently dividing CPU time between runnable tasks.
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Sleep is not processor-intensive and allows the processor to idle
Sleep mode is a low-power state for computers and other devices. In sleep mode, the computer's state is preserved in memory, but other components are shut down and do not consume any power. This allows the device to quickly resume operation when needed without having to go through a full boot-up process.
However, it is important to note that sleep mode does not completely shut down all components of the computer. Some parts, such as the CPU, may remain active or in a low-power state. The specific behaviour of the CPU during sleep mode can vary depending on the device and its power settings.
In some cases, the CPU may be powered off completely during sleep mode, particularly in deeper sleep states (such as S2 or greater). This allows the processor to idle and conserve energy. However, in other cases, the CPU may remain active enough to respond to certain events, such as network connections or user input.
There have been reports of high CPU usage during sleep mode on certain Windows operating systems. In some instances, users have observed 100% CPU usage by the process "explorer.exe" immediately after waking up the computer from sleep mode. This issue does not seem to occur during normal usage and may be related to specific power settings or configurations.
Overall, sleep mode is designed to reduce power consumption and allow the processor to idle while keeping the device in a state that can quickly resume operation. The specific behaviour of the CPU during sleep mode can vary depending on the device, operating system, and power settings.
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Frequently asked questions
No, when a process is sleeping, it is not executing on the CPU, so the amount of CPU time it uses is zero. The operating system is responsible for scheduling threads and can place a thread in a sleep state that can be woken up by a timer.
When a process is sleeping, it is sitting on a pending queue, waiting to be woken up by the operating system (OS). The OS can suspend execution thousands of times a second so that other processes can run.
The CPU does not need to be actively waiting during sleep mode. The system hardware waits and sends an interrupt to the CPU when it needs to wake up.
