Does a thread waiting on IO also block a core?

Question

In the synchronous/blocking model of computation we usually say that a thread of execution will wait (be blocked) while it waits for an IO task to complete.

Answer 1

If we assume it's not async, then I would say, in that case, your thread owning the thread would be put to the waiting queue for sure and the state would be "waiting".

Context-switching wise, IMO, it may need a little bit more explanation since the term context-switch can mean/involve many things (swapping in/out, page table updates, register updates, etc). Depending on the current state of execution, potentially, a second thread that belongs to the same process might be scheduled to run whilst the thread that was blocked on the IO operation is still waiting.

For example, then context-switching would most likely be limited to changing register values on the CPU regarding core (but potentially the owning process might even get swapped-out if there's no much memory left).

Answer 2

A CPU core is normally not dedicated to one particular thread of execution. The kernel is constantly switching processes being executed in and out of the CPU. The process currently being executed by the CPU is in the "running" state. The list of processes waiting for their turn are in a "ready" state. The kernel switches these in and out very quickly. Modern CPU features (multiple cores, simultaneous multithreading, etc.) try to increase the number of threads of execution that can be physically executed at once.

If a process is I/O blocked, the kernel will just set it aside (put it in the "waiting" state) and not even consider giving it time in the CPU. When the I/O has finished, the kernel moves the blocked process from the "waiting" state to the "ready" state so it can have its turn ("running") in the CPU.

So your blocked thread of execution blocks only that: the thread of execution. The CPU and the CPU cores continue to have other threads of execution switched in and out of them, and are not idle.

Answer 3

no,in java , block thread did't participate scheduling

Answer 4

For most programming languages, used in standard ways, then the answer is that it will block your thread, but not your CPU.

You would need to explicitely reserve a CPU for a particular thread (affinity) for 1 thread to block an entire CPU. To be more explicit, see this question:

You could call the SetProcessAffinityMask on every process but yours with a mask that excludes just the core that will "belong" to your process, and use it on your process to set it to run just on this core (or, even better, SetThreadAffinityMask just on the thread that does the time-critical task).