3 ms·

In a sleep state there is no kernel running, it’s a hardware feature that stops CPU execution and possibly more depending on the sleep state.

rcxdude 3y ago
The kernel is still heavily involved in the process of going to sleep: it needs to coordinate the process, which includes freezing userspace execution. So it can quite easily unfreeze only a portion of the userspace. The main trick is making sure that the unfrozen bits aren't depending on the frozen bits, which is probably hard in general thanks to locks.
- arghwhat 3y ago
  Userspace locks are generally per process (or process group), so it shouldn't matter. There can be IPC dependencies and file locks and what not, but none of that matters if you only run something you control and therefore can ensure have no odd dependencies.
arghwhat 3y ago
Sleep states are not a switch you just flick - that be equivalent to you pulling the power cord. Even in the highest, most power hungry sleep states the kernel has to pause everything first, power down all the other hardware (otherwise you'd still have you WiFi card and GPU running full tilt), configure what will interrupt the CPU sleep and finally start it. On the other end it has to set everything back up, reinitialize all the other hardware to be in the same state as before, and finally start running user space again - pretending nothing happened even though the foundation was removed and rebuilt. The only real difference to full hibernation - where the machine is powered off entirely and the kernel restores system state from a snapshot it wrote to disk - is that the RAM state sticks around and that the CPU executes the kernel again on wake instead of dropping to the firmware boot sequence. Without that boot sequence, hibernate would basically be indistinguishable from suspend - an Phoronix article some years back talked about how Intel tuned the kernel to get from cold boot to full interactive UI in 300ms for automotive usecases.
- MaxikCZ 3y ago
  > boot to full interactive UI in 300ms for automotive usecases. Amazing. In the meantime latest Skoda infotainments are starting up when they detect a key approaching the car to be at least somewhat ready by the time you start the car and drive. My friend states that for switching off a "simulate engine noise inside cabin" he need to let infotaintment "warm up" for another minute or two, before it becomes responsive enough to actually be able to turn that off.

rcxdude 3y ago

The kernel is still heavily involved in the process of going to sleep: it needs to coordinate the process, which includes freezing userspace execution. So it can quite easily unfreeze only a portion of the userspace. The main trick is making sure that the unfrozen bits aren't depending on the frozen bits, which is probably hard in general thanks to locks.

arghwhat 3y ago
Userspace locks are generally per process (or process group), so it shouldn't matter. There can be IPC dependencies and file locks and what not, but none of that matters if you only run something you control and therefore can ensure have no odd dependencies.

arghwhat 3y ago

Sleep states are not a switch you just flick - that be equivalent to you pulling the power cord. Even in the highest, most power hungry sleep states the kernel has to pause everything first, power down all the other hardware (otherwise you'd still have you WiFi card and GPU running full tilt), configure what will interrupt the CPU sleep and finally start it. On the other end it has to set everything back up, reinitialize all the other hardware to be in the same state as before, and finally start running user space again - pretending nothing happened even though the foundation was removed and rebuilt. The only real difference to full hibernation - where the machine is powered off entirely and the kernel restores system state from a snapshot it wrote to disk - is that the RAM state sticks around and that the CPU executes the kernel again on wake instead of dropping to the firmware boot sequence. Without that boot sequence, hibernate would basically be indistinguishable from suspend - an Phoronix article some years back talked about how Intel tuned the kernel to get from cold boot to full interactive UI in 300ms for automotive usecases.

MaxikCZ 3y ago
> boot to full interactive UI in 300ms for automotive usecases. Amazing. In the meantime latest Skoda infotainments are starting up when they detect a key approaching the car to be at least somewhat ready by the time you start the car and drive. My friend states that for switching off a "simulate engine noise inside cabin" he need to let infotaintment "warm up" for another minute or two, before it becomes responsive enough to actually be able to turn that off.