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SubjectRe: O_DIRECT question
Viktor wrote:
> OK, madvise() used with mmap'ed file allows to have reads from a file
> with zero-copy between kernel/user buffers and don't pollute cache
> memory unnecessarily. But how about writes? How is to do zero-copy
> writes to a file and don't pollute cache memory without using O_DIRECT?
> Do I miss the appropriate interface?

Not only that but mmap/madvise do not allow you to perform async io.
You still have to just try and touch the memory and take the page fault
to cause a read. This is unacceptable to an application that is trying
to manage multiple IO streams and keep the pipelines full; it needs to
block only when there is no more work it can do.

Even with only a single io stream the application needs to keep one side
reading and one side writing. To make this work without O_DIRECT would
require a method to asynchronously fault pages in, and the kernel would
have to utilize that method when processing aio writes so as not to
block the calling process if the buffer it asked to write is not in
memory.

I was rather disappointed years ago by NT's lack of an async page fault
path, which prevented me from developing an ftp server that could do
zero copy IO, but still use the filesystem cache and avoid NT's version
of O_DIRECT.


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