As i know that Linux implements a fair scheduler and Windows System
implements the Round-robin (RR) schedulers for threads scheduling,
Both Linux and Windows use priority-based, preemptive thread schedulers. Fairness matters but it's not, strictly speaking, the objective. Exactly how these scheduler work depends on the version and the flavor (client vs. server) of the system. Generally, thread schedulers are designed to maximize responsiveness and alleviate scheduling hazards such as inversion and starvation. Although some scheduling decisions are made in a round-robin fashion, there are situations in which the scheduler may insert the preempted thread at the front of the queue rather than at the back.
each thread has a time slice for its execution.
The time slice (or quantum) is really more like a guideline than a rule. It's important to understand that a time slice is divisible and it equals some variable number of clock cycles. The scheduler charges CPU usage in terms of clock cycles, not time slices. A thread may run for more than a time slice (e.g., a time slice and a half). A thread may also voluntarily relinquish the rest of its time slice. This is possible because the only way for a thread to relinquish its time slice is by performing a system call (sleep, yield, acquire lock, request synchronous I/O). All of these are privileged operations that cannot be performed in user-mode (otherwise, a thread can go to sleep without telling the OS!). The scheduler can change the state of the thread from "ready" to "waiting" and schedule some other ready thread to run. If a thread relinquishes the rest of its time slice, it will not be compensated the next time it is scheduled to run.
One particularly interesting case is when a hardware interrupt occurs while a thread is running. In this case, the processor will automatically switch to the interrupt handler, forcibly preempting the thread even if its time slice has not finished yet. In this case, the thread will not be charged for the time it takes to handle the interrupt. Note that the interrupt handler would be indeed utilizing the CPU. By the way, the overhead of context switching itself is also not charged towards any time slice. Moreover, on Windows, the fact that a thread is running in user-mode or kernel-mode by itself does not have an impact on its priority or time slice. On Linux, the scheduler is invoked at specific places in the kernel to avoid starvation (kernel preemption implemented in Linux 2.5+).
So the CPU usage will be 100%? And how is the thread scheduled? Will
thread A use up all its time and then schedule out?
It's easy to answer these questions now. When a thread goes to sleep, the other gets scheduled. Note that this happens even if the threads have different priorities.
If i got a thread running, and blocked by some
condition(e.g network). Will the CPU 100% will affect the wakeup time
of this thread? For example, another thread may running in its time
window and will not schedule out by the OS?
Linux and Windows schedulers implement techniques to enable threads that are waiting on I/O operations to "wake up quickly" and get higher chances of being scheduled soon. For example, on Windows, the priority of a thread waiting on an I/O operation may be boosted a bit when the I/O operation completes. This means that it can preempt another running thread before finishing its time slice, even if both threads had the same priorities initially. When a boosted-priority thread wakes up, its original priority is restored.
