On Thu, Mar 25, 2010 at 08:20:04PM +0900, KOSAKI Motohiro wrote: > > 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. > Ok, I cannot apologise for this enough. Since that paper was published, further work showed that the equation could be much improved. As part of that, I updated the equation to the following; double index = 1 - ( (1 + ((double)info->free_pages / requested)) / info->free_blocks_total); or when approximated to three decimal places int index = 1000 - ( (1000 + ( info->free_pages * 1000 / requested)) / info->free_blocks_total); Your analysis of the paper is perfect. When slotted into a driver program with your example figures, I get the following results old equation = 0.500000 current equation = 0.250000 integer approximation = 250 The code as-is is correct and is what I intended. My explanation on the other hand sucks and I should have remembered that I updated equation since I published that paper 2 years ago. Again, I am extremely sorry for misleading you. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>