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| From | Martin Gregorie <martin@address-in-sig.invalid> |
| Newsgroups | comp.lang.java.programmer |
| Subject | Re: O.T. optimising file placement |
| Date | Fri, 24 Feb 2012 20:21:50 +0000 (UTC) |
| Organization | UK Free Software Network |
| Lines | 46 |
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On Thu, 23 Feb 2012 21:45:33 -0800, Patricia Shanahan wrote: > 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. > Fair point, and I've seen blinding speed from reads where the disk drivers used track reads, but it still doesn't affect my point that there's still only one I/O request in the queue per active single threaded process. Head movement optimisation is simply sidestepped in this case. > 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. > Again agreed: its fair to regard a write as complete from the program's POV as soon as it can reread the block/record - something that many indexed sequential access schemes need to do to re-establish a 'current record' pointer. -- martin@ | Martin Gregorie gregorie. | Essex, UK org |
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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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