Re: Better integration of compression with the broader linux-mm

[Minchan Kim <minchan@kernel.org>]

On Fri, Feb 22, 2013 at 09:15:28AM +0800, Ric Mason wrote:
> On 02/22/2013 08:40 AM, Minchan Kim wrote:
> >On Thu, Feb 21, 2013 at 12:49:21PM -0800, Dan Magenheimer wrote:
> >>Hi Mel, Rik, Hugh, Andrea --
> >>
> >>(Andrew and others also invited to read/comment!)
> >>
> >>In the last couple of years, I've had conversations or email
> >>discussions with each of you which touched on a possibly
> >>important future memory management policy topic.  After
> >>giving it some deep thought, I wonder if I might beg for
> >>a few moments of your time to think about it with me and
> >>provide some feedback?
> >>
> >>There are now three projects that use in-kernel compression
> >>to increase the amount of data that can be stored in RAM
> >>(zram, zcache, and now zswap).  Each uses pages of data
> >>"hooked" from the MM subsystem, compresses the pages of data
> >>(into "zpages"), allocates pageframes from the MM subsystem,
> >>and uses those allocated pageframes to store the zpages.
> >>Other hooks decompress the data on demand back into pageframes.
> >>Any pageframes containing zpages are managed by the
> >>compression project code and, to the MM subsystem, the RAM
> >>is just gone, the same as if the pageframes were absorbed
> >>by a RAM-voracious device driver.
> >>
> >>Storing more data in RAM is generally a "good thing".
> >>What may be a "bad thing", however, is that the MM
> >>subsystem is losing control of a large fraction of the
> >>RAM that it would otherwise be managing.  Since it
> >>is MM's job to "load balance" different memory demands
> >>on the kernel, compression may be positively improving
> >>the efficiency of one class of memory while impairing
> >>overall RAM "harmony" across the set of all classes.
> >>(This is a question that, in some form, all of you
> >>have asked me.)
> >>
> >>In short, the issue becomes: Is it possible to get the
> >>"good thing" without the "bad thing"?  In other words,
> >>is there a way to more closely integrate the management
> >>of zpages along with the rest of RAM, and ensure that
> >>MM is responsible for both?  And is it possible to do
> >>this without a radical rewrite of MM, which would never
> >>get merged?  And, if so... a question at the top of my
> >>mind right now... how should this future integration
> >>impact the design/redesign/merging of zram/zcache/zswap?
> >>
> >>So here's what I'm thinking...
> >>
> >>First, it's important to note that currently the only
> >>two classes of memory that are "hooked" are clean
> >>pagecache pages (by zcache only) and anonymous pages
> >>(by all three).  There is potential that other classes
> >>(dcache?) may be candidates for compression in the future
> >>but let's ignore them for now.
> >>
> >>Both "file" pages and "anon" pages are currently
> >>subdivided into "inactive" and "active" subclasses and
> >>kswapd currently "load balances" the four subclasses:
> >>file_active, file_inactive, anon_active, and anon_inactive.
> >>
> >>What I'm thinking is that compressed pages are really
> >>just a third type of subclass, i.e. active, inactive,
> >>and compressed ("very inactive").  However, since the
> >>size of a zpage varies dramatically and unpredictably --
> >>and thus so does the storage density -- the MM subsystem
> >>should care NOT about the number of zpages, but the
> >>number of pageframes currently being used to store zpages!
> >>
> >>So we want the MM subsystem to track and manage:
> >>
> >>1a) quantity of pageframes containing file_active pages
> >>1b) quantity of pageframes containing file_inactive pages
> >>1c) quantity of pageframes containing file_zpages
> >>2a) quantity of pageframes containing anon_active pages
> >>2b) quantity of pageframes containing anon_inactive pages
> >>2c) quantity of pageframes containing anon_zpages
> >>
> >>For (1a/2a) and (1b/2b), of course, quantity of pageframes
> >>is exactly the same as the number of pages, and the
> >>kernel already tracks and manages these.  For (1c/2c)
> >>however, MM only need care about the number of pageframes, not
> >>the number of zpages.  It is the MM-compression sub-subsystem's
> >>responsibility to take direction from the MM subsystem as
> >>to the total number of pageframes it uses... how (and how
> >>efficiently) it stores zpages in that number of pageframes
> >>is its own business.  If MM tells MM-compression to
> >>reduce "quantity of pageframes containing anon_zpages"
> >>it must be able to do that.
> >>
> >>OK, does that make sense?  If so, I have thoughts on
> >I think that's a good idea.
> >MM can give general API like alloc_pages(GFP_ZSPAGE) and put together
> >sub pages of zspage into LRU_[FILE|ANON]_ZPAGES which would be
> >zone/node aware as well as system-wide LRU.
> >
> >Each sub pages could have a function pointer in struct page somewhere.
> >which would be each MM-compression subsystem's reclaim function.
> >So MM can ask to MM-compression subsystem to reclaim the page
> >when needs happens.
> 
> Why need function pointer in struct page? Since zspages are on
> LRU_[FILE|ANON]_ZPAGES, page reclaim subsystem call reclaim them
> directly.

It would be a subpage of zspage and zspage format might be different in each
MM-compression subsystem so MM layter can't reclaim them without helping from
MM-compression subsytsem, IMHO.

> 
> >
> >It can remove MM-compression's own policy and can add unified abstration
> >layer from MM. Of course, MM can get a complete control.
> >
> >>a more detailed implementation, but will hold that
> >>until after some discussion/feedback.
> >>
> >>Thanks in advance for any time you can spare!
> >>Dan
> >>
> >>--
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> >>see: http://www.linux-mm.org/ .
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> 
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-- 
Kind regards,
Minchan Kim

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