HDD seek time isn't free

By: rwessel (rwessel.delete@this.yahoo.com), May 1, 2021 2:57 pm
Room: Moderated Discussions
Tim Mc (timcaffrey.delete@this.aol.com) on May 1, 2021 2:42 pm wrote:
> Yuhong Bao (yuhongbao_386.delete@this.hotmail.com) on May 1, 2021 2:21 pm wrote:
> > Mark Roulo (nothanks.delete@this.xxx.com) on May 1, 2021 2:12 pm wrote:
> > > Yuhong Bao (yuhongbao_386.delete@this.hotmail.com) on May 1, 2021 1:01 pm wrote:
> > > > The fun thing is that 4K pages probably used to be too large. On a 80386, just 8 tasks would consume
> > > > at least 64k and probably 128k just for the page tables alone. (80386 page tables were two levels)
> > >
> > > What assumptions are you making about hard drive seek times? And the amount
> > > of DRAM to the number or processes more-or-less running at one time?
> > >
> > > 3,000 RPM seems like a reasonable guess for late-1980s hard drives, so 50 Hz.
> > >
> > > Halving the page size would (roughly) double the number of seeks to page in (and
> > > out) the same amount of memory. Have you measured (or modeled) this tradeoff?
> > >
> > >
> >
> > I am particularly thinking of PCs with 4MB RAM which used to be common until 1995 or so.
>
> MFM hard drives were 3600 RPM, 3-5ms seek time (but faster than that for head switching), and
> typically 2:1 interleaving (by 1987, original XT hard drive were formatted at 5:1 interleaving IIRC),
> 17 sectors per track. So best case was a transfer rate of ~260K bytes/sec. Including seek time,
> rotational latency, and 4K pages that span heads/cylinders would effectively reduce the transfer rate significantly.
> By 1995 I think the transfer rate had increased to about 13 MB/s, but the seek times were about the same.
> Tracks held more sectors though, and disk spun faster (5400 or 7200 RPM) so that helped.


Mid-90s seek times were more in the 15ms range. The CMI-6426 on the original AT was 39ms.
< Previous Post in ThreadNext Post in Thread >
TopicPosted ByDate
4K pages probably used to be too largeYuhong Bao2021/05/01 01:01 PM
  HDD seek time isn't freeMark Roulo2021/05/01 02:12 PM
    HDD seek time isn't freeYuhong Bao2021/05/01 02:21 PM
      HDD seek time isn't freeTim Mc2021/05/01 02:42 PM
        HDD seek time isn't freerwessel2021/05/01 02:57 PM
  4K pages probably used to be too largeBen LaHaise2021/05/02 10:45 AM
    VAX was 512 (in 1977) (NT)anonymous22021/05/02 08:36 PM
      FWIW, S/370 offered a choice of 2K and 4K (NT)rwessel2021/05/03 05:09 AM
      DEC's earliest PDP-11 disks were 512 (in 1971)John Yates2021/05/03 01:53 PM
    4K pages probably used to be too largeanon22021/05/03 01:17 AM
      4K pages probably used to be too largeBen LaHaise2021/05/03 05:36 PM
        Morotola 680x0 series page sizesBen LaHaise2021/05/03 05:51 PM
        4K pages probably used to be too largeanon22021/05/03 06:39 PM
          4K pages probably used to be too largeanon22021/05/03 08:51 PM
        4K pages probably used to be too largeYuhong Bao2021/05/03 10:51 PM
      4K pages probably used to be too largewumpus2021/05/05 09:06 AM
        4K pages probably used to be too largeanon22021/05/05 04:04 PM
          4K pages probably used to be too largeChester2021/05/05 06:45 PM
            4K pages probably used to be too largewumpus2021/05/06 09:06 AM
              Phenom TLB bugHeikki Kultala2021/05/06 12:46 PM
                Phenom TLB bugChester2021/05/06 05:29 PM
        4K pages probably used to be too largeEtienne Lorrain2021/05/06 01:08 AM
          4K pages probably used to be too largeJames2021/05/06 02:36 AM
            4K pages probably used to be too largerwessel2021/05/06 09:32 AM
              Reformatting SCSI disk sector sizeDoug S2021/05/06 11:30 AM
        4K pages probably used to be too largeDavid Hess2021/05/11 08:57 AM
  Page size is more complex/nuancedPaul A. Clayton2021/05/08 10:03 AM
Reply to this Topic
Name:
Email:
Topic:
Body: No Text
How do you spell tangerine? 🍊