Re: [PATCHv11 2/4] zbud: add to mm/

[Mel Gorman <mgorman@suse.de>]

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