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aus+uk / uk.comp.sys.mac / Demand paging

SubjectAuthor
* Demand pagingTimS
`* Re: Demand pagingTheo
 `* Re: Demand pagingTimS
  `* Re: Demand pagingTheo
   `- Re: Demand pagingTimS

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Demand paging

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From: tim@streater.me.uk (TimS)
Newsgroups: uk.comp.sys.mac
Subject: Demand paging
Date: 17 Feb 2024 13:10:34 GMT
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 by: TimS - Sat, 17 Feb 2024 13:10 UTC

Anyone know whether macOS / Win / Lin do demand paging or not, and what they
do if not?

--
Tim

Re: Demand paging

<+Nn*ZkdDz@news.chiark.greenend.org.uk>

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From: theom+news@chiark.greenend.org.uk (Theo)
Newsgroups: uk.comp.sys.mac
Subject: Re: Demand paging
Date: 17 Feb 2024 15:03:42 +0000 (GMT)
Organization: University of Cambridge, England
Message-ID: <+Nn*ZkdDz@news.chiark.greenend.org.uk>
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Originator: theom@chiark.greenend.org.uk ([212.13.197.229])
 by: Theo - Sat, 17 Feb 2024 15:03 UTC

TimS <tim@streater.me.uk> wrote:
> Anyone know whether macOS / Win / Lin do demand paging or not, and what they
> do if not?

Are you referring to demand paging specifically, compared with other
algorithms for swapping? Or disc-backed virtual memory in general?

SFAIK they all (can) use swap, but generally that's a last resort these
days. macOS uses compressed memory pages ahead of swapping to disc - ie
when it needs to swap, it compresses pages and stores the compressed output
to RAM, rather than disc. When the page is needed again it's decompressed.
Linux can also do this ('zram') if configured. It seems like Windows 8.1
also introduced it.

Theo

Re: Demand paging

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From: tim@streater.me.uk (TimS)
Newsgroups: uk.comp.sys.mac
Subject: Re: Demand paging
Date: 17 Feb 2024 16:13:12 GMT
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 by: TimS - Sat, 17 Feb 2024 16:13 UTC

On 17 Feb 2024 at 15:03:42 GMT, "Theo" <theom+news@chiark.greenend.org.uk>
wrote:

> TimS <tim@streater.me.uk> wrote:
>> Anyone know whether macOS / Win / Lin do demand paging or not, and what they
>> do if not?
>
> Are you referring to demand paging specifically, compared with other
> algorithms for swapping? Or disc-backed virtual memory in general?

Yes, demand paging specifically. Such as was introduced IIRC with the Ferranti
Atlas and was certainly what VMS used in the 80s.

--
Tim

Re: Demand paging

<-Nn*7FdDz@news.chiark.greenend.org.uk>

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From: theom+news@chiark.greenend.org.uk (Theo)
Newsgroups: uk.comp.sys.mac
Subject: Re: Demand paging
Date: 17 Feb 2024 16:33:51 +0000 (GMT)
Organization: University of Cambridge, England
Message-ID: <-Nn*7FdDz@news.chiark.greenend.org.uk>
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Originator: theom@chiark.greenend.org.uk ([212.13.197.229])
 by: Theo - Sat, 17 Feb 2024 16:33 UTC

TimS <tim@streater.me.uk> wrote:
> On 17 Feb 2024 at 15:03:42 GMT, "Theo" <theom+news@chiark.greenend.org.uk>
> wrote:
>
> > TimS <tim@streater.me.uk> wrote:
> >> Anyone know whether macOS / Win / Lin do demand paging or not, and what they
> >> do if not?
> >
> > Are you referring to demand paging specifically, compared with other
> > algorithms for swapping? Or disc-backed virtual memory in general?
>
> Yes, demand paging specifically. Such as was introduced IIRC with the Ferranti
> Atlas and was certainly what VMS used in the 80s.

They can, they try not to.

Demand paging is pulling in pages when a page fault is detected,
anticipatory paging tries to pull them in ahead of time so there's no page
fault. You are likely doing both - get a page fault, pull in that page and
the next N pages. The first one is demand paged the others are anticipatory
paged. When you hit the next page it's already in memory so you've avoided
the latency cost of having to pull it in.

Operating systems use a variety of paging algorithms:
https://en.wikipedia.org/wiki/Page_replacement_algorithm

Maybe you have a fancier anticipation algorithm that does some machine
learning voodoo on your workload and figures out which pages to anticipate
so good you never demand page. It's all down to whatever algorithms your
swapper is compiled with (which is static in Windows and Mac, but on Linux
you can usually tweak this stuff).

Theo

Re: Demand paging

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From: tim@streater.me.uk (TimS)
Newsgroups: uk.comp.sys.mac
Subject: Re: Demand paging
Date: 17 Feb 2024 17:23:15 GMT
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 by: TimS - Sat, 17 Feb 2024 17:23 UTC

On 17 Feb 2024 at 16:33:51 GMT, "Theo" <theom+news@chiark.greenend.org.uk>
wrote:

> TimS <tim@streater.me.uk> wrote:
>> On 17 Feb 2024 at 15:03:42 GMT, "Theo" <theom+news@chiark.greenend.org.uk>
>> wrote:
>>
>>> TimS <tim@streater.me.uk> wrote:
>>>> Anyone know whether macOS / Win / Lin do demand paging or not, and what they
>>>> do if not?
>>>
>>> Are you referring to demand paging specifically, compared with other
>>> algorithms for swapping? Or disc-backed virtual memory in general?
>>
>> Yes, demand paging specifically. Such as was introduced IIRC with the Ferranti
>> Atlas and was certainly what VMS used in the 80s.
>
> They can, they try not to.

OK - thanks for that. Things have moved on, obvs.

--
Tim

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