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

[Ric Mason <ric.masonn@gmail.com>]

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 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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