> > On Thu, Mar 25, 2010 at 11:47:17AM +0900, KOSAKI Motohiro wrote:
> > > > 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.
>
> Sorry, My example was wrong. new example is here.
>
> free_pages: 4
> requested: 2
> free_blocks_total: 4
>
> theory: 1 - (TotalFree/SizeRequested)/BlocksFree
> = 1 - (4/2)/4 = 0.5
>
> code : 1000 - ((1000 + 4*1000/2))/4 = 1000 - (1000 + 2000)/4 = 1000/4 = 250
>
> I don't think this is three decimal picking up code. This seems might makes
> lots compaction invocation rather than theory.
>
