On Mon, May 20, 2013 at 02:54:39PM +0100, Mel Gorman wrote: > On Sun, May 19, 2013 at 03:52:19PM -0500, Seth Jennings wrote: > > My first guess is that the external fragmentation situation you are referring to > > is a workload in which all pages compress to greater than half a page. If so, > > then it doesn't matter what NCHUCNKS_ORDER is, there won't be any pages the > > compress enough to fit in the < PAGE_SIZE/2 free space that remains in the > > unbuddied zbud pages. > > > > There are numerous aspects to this, too many to write them all down. > Modelling the external fragmentation one and how it affects swap IO > would be a complete pain in the ass so lets consider the following > example instead as it's a bit clearer. > > Three processes. Process A compresses by 75%, Process B compresses to 15%, > Process C pages compress to 15%. They are all adding to zswap in lockstep. > Lets say that zswap can hold 100 physical pages. > > NCHUNKS == 2 > All Process A pages get rejected. Ah, I think this is our disconnect. Process A pages will not be rejected. They will be stored in a zbud page, and that zbud page will be added to the 0th unbuddied list. This list maintains a list of zbud pages that will never be buddied because there are no free chunks. In other words, changing NCHUNKS has no effect on the acceptable size of allocations. Seth _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
