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

[Ric Mason]

Hi Dan,
On 04/10/2013 04:25 AM, Dan Magenheimer wrote:
> > From: Minchan Kim [mailto:minchan@xxxxxxxxxx]
> > Subject: Re: zsmalloc defrag (Was: [PATCH] mm: remove compressed copy from zram in-memory)
> >
> > Hi Dan,
> >
> > On Mon, Apr 08, 2013 at 09:32:38AM -0700, Dan Magenheimer wrote:
> > > > From: Minchan Kim [mailto:minchan@xxxxxxxxxx]
> > > > 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.
> Have you designed or implemented this yet?  I have a couple
> of concerns:
>
> 1) The handle is transparent to the "user", but it is still a form
>     of a "pointer" to a zpage.  Are you planning on walking zram's
>     tables and changing those pointers?  That may be OK for zram
>     but for more complex data structures than tables (as in zswap
>     and zcache) it may not be as easy, due to races, or as efficient
>     because you will have to walk potentially very large trees.
> 2) Compaction in the kernel is heavily dependent on page migration
>     and page migration is dependent on using flags in the struct page.

Which flag?

>     There's a lot of code in those two code modules and there
>     are going to be a lot of implementation differences between
>     compacting pages vs compacting zpages.
>
> I'm also wondering if you will be implementing "variable length
> zspages".  Without that, I'm not sure compaction will help
> enough.  (And that is a good example of the difference between
> the kernel page compaction design/code and zspage compaction.)
>
> > > particular, I am wondering if your design will also
> > > handle the requirements for zcache (especially for
> > > cleancache pages) and perhaps also for ramster.
> > I don't know requirements for cleancache pages but compaction is
> > general as you know well so I expect you can get a benefit from it
> > if you are concern on memory efficiency but not sure it's valuable
> > to compact cleancache pages for getting more slot in RAM.
> > Sometime, just discarding would be much better, IMHO.
> Zcache has page reclaim.  Zswap has zpage reclaim.  I am
> concerned that these continue to work in the presence of
> compaction.   With no reclaim at all, zram is a simpler use
> case but if you implement compaction in a way that can't be
> used by either zcache or zswap, then zsmalloc is essentially
> forking.

I fail to understand "then zsmalloc is essentially forking.", could you 
explain more?

> > > In https://lkml.org/lkml/2013/3/27/501 I suggested it
> > > would be good to work together on a common design, but
> > > you didn't reply.  Are you thinking that zsmalloc
> > I saw the thread but explicit agreement is really matter?
> > I believe everybody want it although they didn't reply. :)
> >
> > You can make the design/post it or prototyping/post it.
> > If there are some conflit with something in my brain,
> > I will be happy to feedback. :)
> >
> > Anyway, I think my above statement "COMPACTION" would be enough to
> > express my current thought to avoid duplicated work and you can catch up.
> >
> > I will get around to it after LSF/MM.
> >
> > > improvements should focus only on zram, in which case
> > Just focusing zsmalloc.
> Right.  Again, I am asking if you are changing zsmalloc in
> a way that helps zram but hurts zswap and makes it impossible
> for zcache to ever use the improvements to zsmalloc.
>
> If so, that's fine, but please make it clear that is your goal.
>
> > > we may -- and possibly should -- end up with a different
> > > allocator for frontswap-based/cleancache-based compression
> > > in zcache (and possibly zswap)?
> > > I'm just trying to determine if I should proceed separately
> > > with my design (with Bob Liu, who expressed interest) or if
> > > it would be beneficial to work together.
> > Just posting and if it affects zsmalloc/zram/zswap and goes the way
> > I don't want, I will involve the discussion because our product uses
> > zram heavily and consider zswap, too.
> >
> > I really appreciate your enthusiastic collaboration model to find
> > optimal solution!
> My goal is to have compression be an integral part of Linux
> memory management.  It may be tied to a config option, but
> the goal is that distros turn it on by default.  I don't think
> zsmalloc meets that objective yet, but it may be fine for
> your needs.  If so it would be good to understand exactly why
> it doesn't meet the other zproject needs.
>
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