From: Eric Wong <normalperson@...>
Date: 2010-11-07T23:22:58+09:00
Subject: [ruby-core:33092] Re: [Ruby 1.9-Feature#4015] File::DIRECT	Constant for O_DIRECT

Run Paint Run Run <redmine@ruby-lang.org> wrote:
> Last suggestion from me, I promise. ;-)

It's a great one and I second it.  Promoting fadvise use would help to
push kernel hackers to implement/improve support for it.

> around posix_fadvise(2). As this advice is never binding, and
> #respond_to? returns false for :advise on platforms that don't support
> it, it is trivial to write portable code that only invokes #advise
> where supported. Granted, this solution still requires defining
> constants. However, there is no danger of defining them on all
> platforms because platforms that don't support this syscall will raise
> a NotImplementedError for #advise.

I suggest making #advise just noop on platforms where it's not currently
supported to avoid #respond_to? checks.  Since "advice" is exactly that,
the underlying implementation in the kernel never guarantees any effect
(and neither can Ruby).  For example, current versions of Linux just
ignore POSIX_FADV_NOREUSE, but that doesn't stop me from putting it in
my code anyways because it could one day be run on a machine where it is
implemented.

Since Ruby isn't strictly tied to POSIX, it could eventually take some
liberties and affect/influence the userspace buffering behavior, too.

> + * _advice_ is one of the following constants:
> + *  
> + *  * File::POSIX_FADV_NORMAL - No advice to give; the default assumption for 
> + *    an open file.
> + *  * File::POSIX_FADV_SEQUENTIAL - The data will be accessed sequentially: 
> + *    with lower offsets read before higher ones.
> + *  * File::POSIX_FADV_RANDOM - The data will be accessed in random order.
> + *  * File::POSIX_FADV_WILLNEED - The data will be accessed in the near future.
> + *  * FILE::POSIX_FADV_DONTNEED - The data will not be accessed in the near 
> + *    future.
> + *  * File::POSIX_FADV_NOREUSE - The data will only be accessed once.

One thing I like about the "fadvise" gem is that it takes symbolic
arguments (much like the 1.9.2 socket API extensions) so I can use it
like this:

   io.fadvise(0, io.stat.size, :dont_need)

Though I'd probably use ":dontneed" or ":DONTNEED" instead.

-- 
Eric Wong