In article <1992Aug5.120359.16381@klaava.Helsinki.FI>, torvalds@klaava.Helsinki.FI (Linus Benedict Torvalds) writes:
| In article <1992Aug5.102706.15668@colorado.edu> drew@ophelia.cs.colorado.edu (Drew Eckhardt) writes:
| >In article <1992Aug5.023248.1639@uniwa.uwa.edu.au> oreillym@tartarus.uwa.edu.au (Michael O'Reilly) writes:
| >>
| >>The 386 chip has a feature that lets you acomplish the same idea without
| >>any fancy schemes like that. The basic idea is that when a page is
| >>accessed for the first time, a bit is set in the page tables. WHen it is
| >>written, a different bit is set. 
| 
| The accessed bit will be used in the soon-to-be-made (I didn't get them
| together yesterday, but I'll try again today) first patch to 0.97. It's
| very practical, and didn't need any additional state information. While
| the current routine isn't a real LRU (Least Recently Used) which needs
| some actual calculations, it's NRU (Not Recently Used) which is a simple
| approximation that can do with just the one bit that is already provided
| by the hardware.

  Bravo! Someone posted that this was the standard 386 paging scheme,
and I thought I had missed it in the source. And I totally forgot that
there was an access bit in the hardware, although if someone had ask it
as a yes/no question I believe I would have rememberd.

| A closer approximation to LRU would be some aging mechanism (the 386
| page tables even have room for a 3-bit wide aging or somthing like that
| in the unused bits), and it might be worth implementing eventually.

  Now that would be really elegant. I don't know if it would save enough
to justify the page table sweep, but elegant none the less. Actually I
don't know if it would be much better than just using the one bit.

-- 
bill davidsen, GE Corp. R&D Center; Box 8; Schenectady NY 12345
    I admit that when I was in school I wrote COBOL. But I didn't compile.