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SubjectRE: Kernel profile
On Tue, 30 Jul 1996, Ray Van Tassle-CRV004 wrote:

> > I think that was the original intention, but the release function was
> > never written (to much overhead in a function call?). If the list
> > maintained a partial ordering (the reverse order of that mentioned above),
> > then the number of iternations - where the active file structures are
> > examined - could be reduced (as in tty_io.c - simply break out of the
> > loop when the first inactive structure is found).
> >
> >
> > Time to dream of an ideal (over kill) solution?
> > The 'idle' process could zero the structures, and move them from the
> > non-zeroed list to the zeroed one.
> >
> > Clearly, this would be fast in allocating a structure, and reduce the
> > number if iterations where the active file structures are searched.
> >
> > Is this using a sledge hammer to crack a nut?
>
> Actually, you don't need two lists. The idle process ("scavenger") could
> walk the list and move empty ones to the front.
>
> As I said earlier---you'd need to model (or capture) access patterns.
> There's the danger of spending a dollar in order to save a dime.

Excuse for talking before checking the code, but I think two lists would
be simpler and probably faster...

With one list, your "scavenger" need to walk the list, and when it finds
a non-zeroed structure, it needs to zero it then move it to the front of
the list. If you have two lists, scavenger only takes the first of the
non-zeroed list, zero it then puts it at the front of the second list, no
walking necessary.

Secondly, with only one list, you need a way to check if the structure
has been zeroed or not - either add a flag to the structure, or re-use an
already there flag... Either way, it's a flag that needs to be checked
then cleared when a process needs to allocate a structure.

So the one-list solution might use a bit more memory (though not much),
add a flag clear, and slow down the "scavenger" in it's job because it
needs to walk the list.

My .02 $CDN

Christophe Dupre Universite de Montreal
Internet: duprec@jsp.umontreal.ca Qc, Canada
Montreal, Qc, Canada
"Nous ne sommes pas libres de ne pas etre libres, nous sommes obliges de
l'etre" - Fernando Savater

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