Re: zsmalloc defrag (Was: [PATCH] mm: remove compressed copy from zram in-memory)

[Minchan Kim <minchan@kernel.org>]

Hi Seth,

On Tue, Apr 09, 2013 at 03:52:36PM -0500, Seth Jennings wrote:
> On 04/08/2013 08:36 PM, Minchan Kim wrote:
> > On Tue, Apr 09, 2013 at 10:27:19AM +0900, Minchan Kim wrote:
> >> Hi Dan,
> >>
> >> On Mon, Apr 08, 2013 at 09:32:38AM -0700, Dan Magenheimer wrote:
> >>>> From: Minchan Kim [mailto:minchan@kernel.org]
> >>>> Sent: Monday, April 08, 2013 12:01 AM
> >>>> Subject: [PATCH] mm: remove compressed copy from zram in-memory
> >>>
> >>> (patch removed)
> >>>
> >>>> Fragment ratio is almost same but memory consumption and compile time
> >>>> is better. I am working to add defragment function of zsmalloc.
> >>>
> >>> Hi Minchan --
> >>>
> >>> I would be very interested in your design thoughts on
> >>> how you plan to add defragmentation for zsmalloc.  In
> >>
> >> What I can say now about is only just a word "Compaction".
> >> As you know, zsmalloc has a transparent handle so we can do whatever
> >> under user. Of course, there is a tradeoff between performance 
> >> and memory efficiency. I'm biased to latter for embedded usecase.
> >>
> >> And I might post it because as you know well, zsmalloc
> > 
> > Incomplete sentense,
> > 
> > I might not post it until promoting zsmalloc because as you know well,
> > zsmalloc/zram's all new stuffs are blocked into staging tree.
> > Even if we could add it into staging, as you know well, staging is where
> > every mm guys ignore so we end up needing another round to promote it. sigh.
> 
> Yes. The lack of compaction/defragmentation support in zsmalloc has not
> been raised as an obstacle to mainline acceptance so I think we should
> wait to add new features to a yet-to-be accepted codebase.
> 
> Also, I think this feature is more important to zram than it is to
> zswap/zcache as they can do writeback to free zpages.  In other words,
> the fragmentation is a transient issue for zswap/zcache since writeback
> to the swap device is possible.

Other benefit derived from compaction work is that we can pick a zpage
from zspage and move it into somewhere. It means core mm could control
pages in zsmalloc freely.

> 
> Thanks,
> Seth
> 
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-- 
Kind regards,
Minchan Kim

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