[llvm-dev] LLD: Using sendfile(2) to copy file contents

Matt Godbolt via llvm-dev llvm-dev at lists.llvm.org
Mon Jun 6 12:11:52 PDT 2016

On Mon, Jun 6, 2016 at 1:41 PM Rui Ueyama via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> As to leave an opportunity for the kernel, I think mmap+write would be
> enough. Because the kernel knows what address is mmap'ed, it can detect
> that write's source is actually a mmap'ed file and if that's the case it
> can optimize as it does for sendfile. It seems that Linux doesn't do that
> now, though.

Pardon my ignorance here, but how might the kernel in general know what the
"source" of a write is?

Also, in terms of the async_io option, in my (non-llvm) experimentation
with reading very large files the aio subsystem is not well-supported or
optimized  (hence the lack of Glibc support).
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