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

Matt Godbolt via llvm-dev llvm-dev at lists.llvm.org
Mon Jun 6 12:20:19 PDT 2016

Perhaps I misunderstand: the kernel can tell (upon a page fault) what
memory address is being written to, and likewise upon a page fault which
memory has been read from. But it can't put these things together to infer
"process A is reading from X and writing to Y": it sees the reads and
writes in isolation, and indeed only at page granularity and when a page
fault happens.

On Mon, Jun 6, 2016 at 2:18 PM Rui Ueyama <ruiu at google.com> wrote:

> On Mon, Jun 6, 2016 at 12:11 PM, Matt Godbolt <matt at godbolt.org> wrote:
>> 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?
> The kernel knows where all mmap'ed files are mapped. So, it can decides
> whether a memory address is in a mmap'ed region or not, no?
> 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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