Re: [RFC] Optimization for use-once pages

[Rik van Riel]

On Tue, 24 Jul 2001, Stuart MacDonald wrote:
> From: "Rik van Riel" <[e-mail]>
> > Because they occur in a very short interval, an interval MUCH
> > shorter than the time scale in which the VM subsystem looks at
> > referenced bits, etc...
>
> So what you're sayng is that any number of accesses, as long
> as they all occur within the interval < VM subsystem scanning
> interval, are all counted as one?

Actually, the length of this interval could be even smaller
and is often a point of furious debating.

The 2Q algorithm seems to have solved this problem by not
using an interval, but a FIFO queue of small, fixed length.

> > This seems to be generally accepted theory and practice in the
> > field of page replacement.
>
> Okay, just seems odd to me, but IANAVMGuru.

Seems odd at first glance, true.

Let me give you an example:

- sequential access of a file
- script reads the file in 80-byte segments
  (parsing some arcane data structure)
- these segments are accessed in rapid succession
- each 80-byte segment is accessed ONCE

In this case, even though the data is accessed only
once, each page is touched PAGE_SIZE/80 times, with
one 80-byte read() each time.

I hope this example gives you some holding point to
make it easier and grasp this - somewhat counter
intuitive - concept.

regards,

Rik
--
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