From: Run Paint Run Run <redmine@...>
Date: 2010-11-06T03:59:12+09:00
Subject: [ruby-core:33075] [Ruby 1.9-Feature#4015] File::DIRECT Constant for O_DIRECT


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Issue #4015 has been updated by Run Paint Run Run.

File io-advise.patch added

Last suggestion from me, I promise. ;-)

O_DIRECT is a controversial (<http://lkml.org/lkml/2007/1/11/121>) flag, =
and Linus and others recommend (<http://lkml.org/lkml/2007/1/10/233>) usi=
ng madvise(2)/posix_fadvise(2) instead. The latter is, as the name sugges=
ts, part of the POSIX standard, avoiding the current problem of defining =
platform-specific, non-standard flags. Its advantages are summarised in O=
'Reilly's _Linux System Programming_:

<<A handful of common application workloads can readily benefit from a li=
ttle well-
intentioned advice to the kernel. Such advice can go a long way toward mi=
tigating
the burden of I/O. With hard disks being so slow, and modern processors b=
eing so
fast, every little bit helps, and good advice can go a long way.

Before reading a chunk of a file, a process can provide the POSIX_FADV_WI=
LLNEED hint
to instruct the kernel to read the file into the page cache. The I/O will=
 occur asyn-
chronously, in the background. When the application ultimately accesses t=
he file, the
operation can complete without generating blocking I/O.

Conversely, after reading or writing a lot of data=E2=80=94say, while con=
tinuously streaming
video to disk=E2=80=94a process can provide the POSIX_FADV_DONTNEED hint =
to instruct the
kernel to evict the given chunk of the file from the page cache. A large =
streaming
operation can continually fill the page cache. If the application never i=
ntends to
access the data again, this means the page cache will be filled with supe=
rfluous data,
at the expense of potentially more useful data. Thus, it makes sense for =
a streaming
video application to periodically request that streamed data be evicted f=
rom the
cache.

A process that intends to read in an entire file can provide the POSIX_FA=
DV_SEQUENTIAL
hint, instructing the kernel to perform aggressive readahead. Conversely,=
 a process
that knows it is going to access a file randomly, seeking to and fro, can=
 provide the
POSIX_FADV_RANDOM hint, instructing the kernel that readahead will be not=
hing but
worthless overhead.>>

Implementing posix_fadvise(2) would avoid complicating the logic of `open=
` any more, and at the same time provide a more general solution than O_D=
IRECT. The attached patch defines IO#advise as a wrapper around posix_fad=
vise(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 po=
rtable code that only invokes #advise where supported. Granted, this solu=
tion still requires defining constants. However, there is no danger of de=
fining them on all platforms because platforms that don't support this sy=
scall will raise a NotImplementedError for #advise.
----------------------------------------
http://redmine.ruby-lang.org/issues/show/4015

----------------------------------------
http://redmine.ruby-lang.org

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