3 ms·

If you are reading unbuffered (ie. O_DIRECT) then you are reading directly into the memory block the user supplied, so you cannot read ahead - there's nowhere t

• paulsutter 7y ago
  Of course you can read ahead using multiple buffers, you can issue as many reads as you want concurrently
  • caf 7y ago
    I think it is pretty clear I was referring to kernel-mediated readahead. Sure, you can achieve the same thing in userspace using async IO or threads.
    • paulsutter 7y ago
      I think it’s pretty clear that I was referring to reading large files using direct io, and your comment was totally irrelevant
      • caf 7y ago
        Reading large files using direct IO defeats kernel read-ahead, which means you have to take on the complexity of reimplementing it in userspace, or the performance hit of not having it. This is a good reason for programs to use buffered IO even when they are reading a large file once, so yes my comment was entirely on point.
        • paulsutter 7y ago
          Your comments were misleading, even if you want to argue they were on point. I know that you're knowledgable about the kernel, I ready your other comments. The following comments are just not true, except in bizarro world where all code must be in the kernel: - "Readahead... needs to be buffered" - "reading unbuffered (ie. O_DIRECT)... you cannot read ahead" Good application code needs to work around kernel limitations all the time. Your suggestion of fadvise is reasonable, except in practice fadvise rarely makes a measurable difference. The only way to get what you really want is to code it the way you want it, and fortunately, there are multiple ways to do that.