On Thu, May 23, 2013 at 10:00:07AM +0800, Bob Liu wrote: > Hi Mel & Seth, > > On 05/21/2013 04:10 PM, Mel Gorman wrote: > > On Mon, May 20, 2013 at 10:42:25AM -0500, Seth Jennings wrote: > >> 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. > >> > > > > D'oh, good point. Unfortunately, the problem then still exists at the > > writeback end which I didn't bring up in the previous mail. > > What's your opinion if we write back the whole compressed page to swap disk? > I'm not sure how to answer that sensibly. If the compressed page is written to swap, in my opinion then there will be IO :/ . It will be a maximum of two pages of IO with zbud (or zpair or whatever) as currently implemented. With zsmalloc it potentially was more. -- Mel Gorman SUSE Labs _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
