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

[Minchan Kim <minchan@kernel.org>]

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.

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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-- 
Kind regards,
Minchan Kim

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