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Comment by sureglymop | Hacker News Reader

Perhaps I misunderstand something but doesn't reading from a file require a system call? And when there is a system call, the context switches? So wouldn't using multiple threads to read from a file mean that they can't really read in parallel anyway because they block each other when executing that system call?

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System calls aren't context switches. They flip a permission bit in the CPU but don't do the work a context switch involves like modifying the MMU, flushing the TLBs, modifying kernel structures, doing scheduling etc.

Also, modern filing systems are all thread safe. You can have multiple threads reading and even writing in parallel on different CPU cores.

> system call, the context switches

No, there is no separate kernel "executing". When you do a syscall, your thread becomes kernel mode and it executes the function behind the syscall, then when it's done, your thread reverts to user mode.

A context switch is when one thread is being swapped out for another. Now the syscall could internally spawn a thread and context switch to that, but I'm not sure if this happens in read() or any syscall for that matter.

What all other siblings said - syscalls are not context switch, they are called 'mode switch' and it has significantly less impact.

If you open() read-only I don't think it blocks (some other process writing to it might block though).

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