Is the timeslice given to a thread that is waiting on I/O "wasted"?

I'm currently analyzing the pros and cons of writing a server using a threaded model or event driven model. I already know the many cons of the threaded model (does not scale well due to context switching overhead, virtual memory limitations, etc.) but I came upon another one in my analysis and would like to verify that my understanding of threads is correct.

If I have 5 threads, 1 which is doing work (not being blocked), 4 which are being blocked waiting for I/O (for example waiting on data from a socket), isn't the CPU time given to those 4 threads essentially wasted since no work is actually being done (assuming no data arrives)? The timeslice given to those 4 blocked threads is taking away time from the 1 thread actually doing work, 开发者_运维百科correct?

In this case I'm explicitly saying that the socket is a blocking one.

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No. Although it actually depends on the type of OS, type of I/O (polled/DMA) and device driver architecture, most device I/O is performed using DMA + interrupts. In such cases a thread is put into a sleep state until an interrupt is triggered for such I/O operations and scheduler does not visit those threads until their pending I/O is complete. Only polling I/O can cause consumption of CPU, such as PIO mode for hard disks.

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Threads don't need to use their entire timeslice. I don't know the specifics, but if blocked threads even get time, they certainly don't use it all.

Obviously, these details vary platform-to-platform-to-environment-to-etc.