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SubjectRe: Idea for improving linux buffer cache behaviour
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On Sat, Oct 04, 2003 at 10:34:58PM -0700, Mike Fedyk wrote:
> On Sat, Oct 04, 2003 at 03:14:14PM -0400, Rik van Riel wrote:
> > On Sat, 4 Oct 2003, David Ashley wrote:
> >
> > > Forgive me if this has already been thought of, or is obsolete, or is
> > > just plain a bad idea, but here it is:
> >
> > Do you also want an answer if the kernel already does
> > exactly what you are suggesting ? ;)
> >
>
> Then why doesn't it work better?
>
> > > 1) Lowest access count looked at first to toss
> > > 2) If access counts equal, throw out oldest first
> >
> > > The net result is commonly used items you very much want to remain in
> > > cache always quickly get rated very highly as the system is used.
> >
> > Which results in exactly the behaviour you're complaining
> > about ;))
>
> So, you use the system, have glibc loaded, and then play a dvd, and now
> glibc needs to be re-read because it's not in cache.
>
> Why wasn't glibc (one example) kept in cache with the streaming read from
> the dvd?

There may be many reasons here, take a look at how many times the
dvd contents were used. You may get a surprise there.
The number ought to be 1, right? But the burner program may read
smaller chunks or something, causing many references to the same block.

Also, the number-of-references approach has its own problems.
Something that is used a lot for a while will stay in cache for
a long while when no longer used, taking up space. That can be
a problem too - i.e. run some large simulation which fill up
memory for a while, and nothing else stays in cache afterwards.

Helge Hafting
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