Groups > comp.lang.java.programmer > #12291

Re: O.T. optimising file placement

Date	2012-02-23 21:45 -0800
From	Patricia Shanahan <pats@acm.org>
Newsgroups	comp.lang.java.programmer
Subject	Re: O.T. optimising file placement
References	(3 earlier) <f6fak7d0tf9tva9lqh0jtd6imjgm8u5oa3@4ax.com> <ji3q44$le1$1@news.albasani.net> <ji44tm$j5o$3@localhost.localdomain> <4f45a713$0$291$14726298@news.sunsite.dk> <ji6hcc$8j1$3@localhost.localdomain>
Message-ID	<dKmdnbUGUZqfudrSnZ2dnUVZ_g6dnZ2d@earthlink.com> (permalink)

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On 2/23/2012 3:16 PM, Martin Gregorie wrote:
> On Wed, 22 Feb 2012 21:40:19 -0500, Arne Vajhøj wrote:
>
>>
>> Most OS'es support async IO.
>>
> Yes, I know, but its not relevant to a single-threaded process since its
> logic generally requires it to wait for a read or write to complete
> before it continues[1]. Hence my comment that this prevents head movement
> being optimized unless a lot of processes are active because there's only
> one outstanding IOP per process.
>
> [1] unless you're deliberately doing async i/o using poll() or
>      select() (in C) or nio (in Java), in which case the process is often
>      best regarded as a half-way house between single and multi-threaded
>      logic.
>
>

There are some exceptions to this. For example, if you are reading a
file sequentially, the OS may prefetch blocks you have not yet
requested, and have multiple reads outstanding as a result.

Depending on the OS and how the IO is being handled, a write may appear
to be complete from the program's point of view once the data has been
copied to a kernel buffer. The OS may be writing out modified blocks,
including swap space blocks, at any time.

Patricia

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O.T. optimising file placement Roedy Green <see_website@mindprod.com.invalid> - 2012-02-21 09:59 -0800
  Re: O.T. optimising file placement Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-21 18:05 -0800
  Re: O.T. optimising file placement Jeff Higgins <jeff@invalid.invalid> - 2012-02-21 21:41 -0500
    Re: O.T. optimising file placement Jeff Higgins <jeff@invalid.invalid> - 2012-02-21 23:23 -0500
      Re: O.T. optimising file placement Roedy Green <see_website@mindprod.com.invalid> - 2012-02-22 11:15 -0800
        Re: O.T. optimising file placement Jeff Higgins <jeff@invalid.invalid> - 2012-02-22 16:00 -0500
          Re: O.T. optimising file placement Jeff Higgins <jeff@invalid.invalid> - 2012-02-22 16:03 -0500
            Re: O.T. optimising file placement Gene Wirchenko <genew@ocis.net> - 2012-02-22 14:52 -0800
              Re: O.T. optimising file placement Jeff Higgins <jeff@invalid.invalid> - 2012-02-22 18:24 -0500
                Re: O.T. optimising file placement Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-02-22 17:10 -0800
                Re: O.T. optimising file placement Gene Wirchenko <genew@ocis.net> - 2012-02-22 20:02 -0800
        Re: O.T. optimising file placement Lew <noone@lewscanon.com> - 2012-02-22 14:27 -0800
          Re: O.T. optimising file placement Martin Gregorie <martin@address-in-sig.invalid> - 2012-02-23 01:31 +0000
            Re: O.T. optimising file placement Arne Vajhøj <arne@vajhoej.dk> - 2012-02-22 21:40 -0500
              Re: O.T. optimising file placement Martin Gregorie <martin@address-in-sig.invalid> - 2012-02-23 23:16 +0000
                Re: O.T. optimising file placement Patricia Shanahan <pats@acm.org> - 2012-02-23 16:22 -0800
                Re: O.T. optimising file placement Patricia Shanahan <pats@acm.org> - 2012-02-23 21:45 -0800
                Re: O.T. optimising file placement Martin Gregorie <martin@address-in-sig.invalid> - 2012-02-24 20:21 +0000
                Re: O.T. optimising file placement Arne Vajhøj <arne@vajhoej.dk> - 2012-02-24 20:31 -0500
                Re: O.T. optimising file placement Martin Gregorie <martin@address-in-sig.invalid> - 2012-02-25 13:15 +0000
            Re: O.T. optimising file placement Lew <noone@lewscanon.com> - 2012-02-23 16:39 -0800
              Re: O.T. optimising file placement Martin Gregorie <martin@address-in-sig.invalid> - 2012-02-24 01:49 +0000
                Re: O.T. optimising file placement Lew <noone@lewscanon.com> - 2012-02-24 10:50 -0800
                Re: O.T. optimising file placement Martin Gregorie <martin@address-in-sig.invalid> - 2012-02-24 23:59 +0000
              Re: O.T. optimising file placement Arne Vajhøj <arne@vajhoej.dk> - 2012-02-24 20:25 -0500
    Re: O.T. optimising file placement Roedy Green <see_website@mindprod.com.invalid> - 2012-02-22 10:54 -0800

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