Re: aio_read/write versus O_NONBLOCK

phil-news-nospam@xxxxxxxx writes:
On Thu, 15 May 2008 21:56:27 GMT Scott Lurndal <scott@xxxxxxxxxxxxx> wrote:
| phil-news-nospam@xxxxxxxx writes:
|>On Thu, 15 May 2008 18:55:43 GMT Scott Lurndal <scott@xxxxxxxxxxxxx> wrote:
|>| phil-news-nospam@xxxxxxxx writes:
|>|>On Wed, 14 May 2008 00:13:59 -0400 Barry Margolin <barmar@xxxxxxxxxxxx> wrote:
|>|>| In article
|>|>| <fe463120-697d-4100-9ca8-b74083491664@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
|>|>| RazvanD <razvand@xxxxxxxxx> wrote:
|>|>|
|>|>|> Hi!
|>|>|>
|>|>|> Could someone point me to some articles or give me some hints on the
|>|>|> advantages/disadvantages of asynchronous operations on files (aio_read/
|>|>|> aio_read) versus normal operations (read/write) used with O_NONBLOCK
|>|>|> when opening a file.
|>|>|>
|>|>|> aio_read/aio_write are indeed more flexible. But, at least on Linux/
|>|>|> glibc, they are implemented using POSIX threads. Wouldn't a carefully
|>|>|> designed program using O_NONBLOCK for files best a program using
|>|>|> aio_read/aio_write? I think my question should be: are asynchronous I/
|>|>|> O operations only more flexible or are they also faster?
|>|>|
|>|>| I'm not certain about Linux, but on many systems O_NONBLOCK has no
|>|>| effect on ordinary file streams.
|>|>
|>|>It has not had any effect on ordinary file streams in Linux in the programs
|>|>I have written that tried it (a couple of them).
|>|>
|>|
|>| It's pretty much of a waste of time. O_NONBLOCK doesn't make any sense
|>| for file descriptors that can't block on a read (i.e. disk-based files).
|>
|>Could you explain why you believe that to be the case?
|>
|
| It's simple. The disk is always there, so a read must always complete
| in a bounded time. O_NONBLOCK was added to Unix to handle cases where
| a blocking read can be unbounded (serial ports, parallel ports, network
| ports).

But a bounded time is not zero time. There are things that can be done
while a disk is being read.

I don't think you really understand how file-based I/O works in linux.
In the majority of cases, by the time you issue the read(2)/pread(2) system
call, it is likely that the data has already been read from the disk.

In any case, what you want is asychronous I/O, rather than non-blocking I/O.

See aio_read/aio_write/lio_listio.

Btw, if you're copying one file to another, use mmap/madvise
not read/pread/write/pwrite.

scott
.

Relevant Pages

  • Re: aio_read/write versus O_NONBLOCK
    ... The disk is always there, so a read must always complete ... | I don't think you really understand how file-based I/O works in linux. ... Why do you not think I really understand how file-based I/O works in Linux? ...
    (comp.unix.programmer)
  • Re: pitiful performance of an SATA150 drive
    ... On FreeBSD, the disk device nodes provide unbuffered disk access, under the assumption that consumers will be providing their own caching, or they wouldn't be using the direct disk interface. ... In Linux, they probably still maintain the character vs. block distinction, in which case one may well offer significant caching, which will show up on micro-benchmarks that issue small I/O sizes or behave relatively unintelligently. ...
    (freebsd-current)
  • Re: Programming Question: non-blocking I/O
    ... DM> I'm porting an app from Windows to Linux and have not yet found how to ... DM> How is this done under Linux? ... now any sort of background processing for disk I/O. ... I/O is always synchronous and effectively instantaneous -- user process ...
    (comp.os.linux.misc)
  • Re: Losing clock ticks (Was: Severe time drift for Undisciplined Local Clock)
    ... ticks if it is doing other things that require disk I/O.) ... It's a known problem with some versions of both Linux and Windows. ...
    (comp.protocols.time.ntp)
  • Re: Linux executable picks up FreeBSD library over linux one and breaks
    ... But if it is in a subdirectory where no FreeBSD lib resides, it is ok (the linux browser sets LD_LIBRARY_PATH in the start script to the right path). ... Have a look how the native browser works, the private libs are not in ldconfig either and the browser start script sets the library path for the browser binary. ... don't care for ports to do at all. ... install libs or hide the libs in special dirs), ...
    (freebsd-hackers)