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

Matt Godbolt via llvm-dev llvm-dev at lists.llvm.org
Mon Jun 6 12:50:49 PDT 2016

Ah! I understand! Thank you for clarifying! I agree; this indeed is
something which seems like the kernel could accelerate.

On Mon, Jun 6, 2016 at 2:34 PM Rui Ueyama <ruiu at google.com> wrote:

> What I mean is doing something like this.
>   int in = open(infile, O_RDONLY, 0);
>   int out = open(outfile, O_WRONLY | O_CREAT, 0644);
>   void *buf = mmap(NULL, filesize, PROT_READ, MAP_PRIVATE, in, 0);
>   write(out, buf, filesize);
> By write, I mean write system call and not a general read or write.
> On Mon, Jun 6, 2016 at 12:20 PM, Matt Godbolt <matt at godbolt.org> wrote:
>> 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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