> > > On Tue, Mar 23, 2010 at 09:22:04AM +0900, KOSAKI Motohiro wrote:
> > > > > > > + /*
> > > > > > > + * Index is between 0 and 1 so return within 3 decimal places
> > > > > > > + *
> > > > > > > + * 0 => allocation would fail due to lack of memory
> > > > > > > + * 1 => allocation would fail due to fragmentation
> > > > > > > + */
> > > > > > > + return 1000 - ( (1000+(info->free_pages * 1000 / requested)) / info->free_blocks_total);
> > > > > > > +}
> > > > > >
> > > > > > Dumb question.
> > > > > > your paper (
http://portal.acm.org/citation.cfm?id=1375634.1375641) says
> > > > > > fragmentation_index = 1 - (TotalFree/SizeRequested)/BlocksFree
> > > > > > but your code have extra '1000+'. Why?
> > > > >
> > > > > To get an approximation to three decimal places.
> > > >
> > > > Do you mean this is poor man's round up logic?
> > >
> > > Not exactly.
> > >
> > > The intention is to have a value of 968 instead of 0.968231. i.e.
> > > instead of a value between 0 and 1, it'll be a value between 0 and 1000
> > > that matches the first three digits after the decimal place.
> >
> > Let's consider extream case.
> >
> > free_pages: 1
> > requested: 1
> > free_blocks_total: 1
> >
> > frag_index = 1000 - ((1000 + 1*1000/1))/1 = -1000
> >
> > This is not your intension, I guess.
>
> Why not?
>
> See this comment
>
> /* Fragmentation index only makes sense when a request would fail */
>
> In your example, there is a free page of the requested size so the allocation
> would succeed. In this case, fragmentation index does indeed go negative
> but the value is not useful.
>
> > Probably we don't need any round_up/round_down logic. because fragmentation_index
> > is only used "if (fragindex >= 0 && fragindex <= 500)" check in try_to_compact_pages().
> > +1 or -1 inaccurate can be ignored. iow, I think we can remove '1000+' expression.
> >
>
> This isn't about rounding, it's about having a value that normally is
> between 0 and 1 expressed as a number between 0 and 1000 because we
> can't use double in the kernel.
