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

From: Bob Liu
Date: Wed May 22 2013 - 22:01:27 EST

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?

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