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

From: Seth Jennings
Date: Mon May 20 2013 - 11:42:41 EST

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.


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