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Asked: 2026-05-11T03:56:48+00:00

Can you make file copying faster through multiple threading? Edit : To clarify, suppose

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Can you make file copying faster through multiple threading?

Edit: To clarify, suppose you were implementing CopyFile(src, tgt). It seems logical that under certain circumstances you could use multiple threads to make it go faster.

Edit Some more thoughts:

Naturally, it depends on the HW/storage in question.

If you’re copying from one disk to another, for example, it’s pretty clear that you can read/write at the same time using two threads, thus saving the performance cost of the fastest of the two (usually reading). But you don’t really need multiple threads for reading/writing in parallel, just async-IO.

But if async-IO can really speed things up (up to 2x) when reading/writing from different disks, why isn’t this the default implementation of CopyFile? (or is it?)

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1 Answer

  1. You can see a benefit particularly if the files are on different devices in which case the I/O can be very effectively overlapped.

    However, there are also cases where where you could easily cause thrashing of the hardware, so I don’t think it’s an optimization that should be taken lightly.

    As far as the additional question you added:

    But if async-IO can really speed things up (up to 2x) when reading/writing from different disks, why isn’t this the default implementation of CopyFile? (or is it?)

    I don’t know the internals of CopyFile(), but I wouldn’t be surprised if they do not do it for a couple reasons:

    1. if they were to implement it using an additional thread (or threads) that might be a bit more intrusive to a process than is appropriate (especially if the process is single threaded to this point)
    2. if they were to try to implement it using asynchronous I/O with a single thread (as ChrisW indicated is a possibility), they might be as likely to cause thrashing problems as improve performance. It might not be easy to generically determine when you’ll get a benefit as opposed to a detriment.

    This is not to say it couldn’t or shouldn’t be done (or even that it isn’t done – I don’t know) – these are just a couple possible reasons why it might not be done.

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