4K pages probably used to be too large

By: anon2 (anon.delete@this.anon.com), May 3, 2021 8:51 pm
Room: Moderated Discussions
anon2 (anon.delete@this.anon.com) on May 3, 2021 6:39 pm wrote:
> Ben LaHaise (bcrl.delete@this.kvack.org) on May 3, 2021 5:36 pm wrote:
> > anon2 (anon.delete@this.anon.com) on May 3, 2021 1:17 am wrote:
> > > Ben LaHaise (bcrl.delete@this.kvack.org) on May 2, 2021 10:45 am 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)
> > > >
> > > > The National Semiconductor 32016 had 512 byte page sizes. The problem is that overhead of small
> > > > page sizes becomes excessive as soon as you have more than a couple of megabytes of memory.
> > > > With 16MB of RAM and 512 byte pages that works out to 65536 pages for which data structures
> > > > to track all the individual pages are needed.
> > >
> > > That's what Linux does, but it is not necessarily the best size/speed tradeoff for very
> > > small memory systems. Tracking per page data with 4k pages and 64 bytes per page is
> > > still 25% the overhead of your "unviable" solution, which doesn't sound great when you
> > > put it that way. That being said I don't think it's necessarily bad at all.
> >
> > Ehh? 4KB pages with 64 bytes per struct page in Linux is 1.56% of memory used for overhead. That's
> > an order of magnitude less than the 12-25% memory overhead as would be the case with 512 byte pages.
>
> I was responding to your miscalculated numbers, and using 32 bytes per page
> because that's what you would use on a tiny memory constrained system.
>
> >
> > Then there's the overhead of the page tables themselves.
>
> I acknowledged that.
>
> > Using 512 byte pages you'd need 5 levels of page
> > tables to cover a 32 bit address space.
>
> Wrong numbers again. 4 levels will cover it. 3 levels (28 bits) would be fine for a tiny memory system.

30 bits of course for 3 levels, now I give the wrong numbers! But still, it should be intuitively obvious that 4KB (12 bits) is a long way off doubling index size of 9 bits or even 10 and 7, so you would never need more than 2x the levels.
< 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
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