Re: zcache+zram working together?

[Minchan Kim <minchan@kernel.org>]

On Wed, Feb 20, 2013 at 02:00:28PM +0800, Will Huck wrote:
> On 02/20/2013 01:53 PM, Minchan Kim wrote:
> >On Wed, Feb 20, 2013 at 11:47:32AM +0900, Kyungmin Park wrote:
> >>On Wed, Feb 20, 2013 at 9:06 AM, Minchan Kim <minchan@kernel.org> wrote:
> >>>On Sat, Feb 16, 2013 at 04:15:41PM +0800, Simon Jeons wrote:
> >>>>On 12/11/2012 02:42 PM, Minchan Kim wrote:
> >>>>>On Fri, Dec 07, 2012 at 01:31:35PM -0800, Dan Magenheimer wrote:
> >>>>>>Last summer, during the great(?) zcache-vs-zcache2 debate,
> >>>>>>I wondered if there might be some way to obtain the strengths
> >>>>>>of both.  While following Luigi's recent efforts toward
> >>>>>>using zram for ChromeOS "swap", I thought of an interesting
> >>>>>>interposition of zram and zcache that, at first blush, makes
> >>>>>>almost no sense at all, but after more thought, may serve as a
> >>>>>>foundation for moving towards a more optimal solution for use
> >>>>>>of "adaptive compression" in the kernel, at least for
> >>>>>>embedded systems.
> >>>>>>
> >>>>>>To quickly review:
> >>>>>>
> >>>>>>Zram (when used for swap) compresses only anonymous pages and
> >>>>>>only when they are swapped but uses the high-density zsmalloc
> >>>>>>allocator and eliminates the need for a true swap device, thus
> >>>>>>making zram a good fit for embedded systems.  But, because zram
> >>>>>>appears to the kernel as a swap device, zram data must traverse
> >>>>>>the block I/O subsystem and is somewhat difficult to monitor and
> >>>>>>control without significant changes to the swap and/or block
> >>>>>>I/O subsystem, which are designed to handle fixed block-sized
> >>>>>>data.
> >>>>>>
> >>>>>>Zcache (zcache2) compresses BOTH clean page cache pages that
> >>>>>>would otherwise be evicted, and anonymous pages that would
> >>>>>>otherwise be sent to a swap device.  Both paths use in-kernel
> >>>>>>hooks (cleancache and frontswap respectively) which avoid
> >>>>>>most or all of the block I/O subsystem and the swap subsystem.
> >>>>>>Because of this and since it is designed using transcendent
> >>>>>>memory ("tmem") principles, zcache has a great deal more
> >>>>>>flexibility in control and monitoring.  Zcache uses the simpler,
> >>>>>>more predictable "zbud" allocator which achieves lower density
> >>>>>>but provides greater flexibility under high pressure.
> >>>>>>But zcache requires a swap device as a "backup" so seems
> >>>>>>unsuitable for embedded systems.
> >>>>>>
> >>>>>>(Minchan, I know at one point you were working on some
> >>>>>>documentation to contrast zram and zcache so you may
> >>>>>>have something more to add here...)
> >>>>>>
> >>>>>>What if one were to enable both?  This is possible today with
> >>>>>>no kernel change at all by configuring both zram and zcache2
> >>>>>>into the kernel and then configuring zram at boottime.
> >>>>>>
> >>>>>>When memory pressure is dominated by file pages, zcache (via
> >>>>>>the cleancache hooks) provides compression to optimize memory
> >>>>>>utilization.  As more pressure is exerted by anonymous pages,
> >>>>>>"swapping" occurs but the frontswap hooks route the data to
> >>>>>>zcache which, as necessary, reclaims physical pages used by
> >>>>>>compressed file pages to use for compressed anonymous pages.
> >>>>>>At this point, any compressions unsuitable for zbud are rejected
> >>>>>>by zcache and passed through to the "backup" swap device...
> >>>>>>which is zram!  Under high pressure from anonymous pages,
> >>>>>>zcache can also be configured to "unuse" pages to zram (though
> >>>>>>this functionality is still not merged).
> >>>>>>
> >>>>>>I've plugged zcache and zram together and watched them
> >>>>>>work/cooperate, via their respective debugfs statistics.
> >>>>>>While I don't have benchmarking results and may not have
> >>>>>>time anytime soon to do much work on this, it seems like
> >>>>>>there is some potential here, so I thought I'd publish the
> >>>>>>idea so that others can give it a go and/or look at
> >>>>>>other ways (including kernel changes) to combine the two.
> >>>>>>
> >>>>>>Feedback welcome and (early) happy holidays!
> >>>>>Interesting, Dan!
> >>>>>I would like to get a chance to investigate it if I have a time
> >>>>>in future.
> >>>>>
> >>>>>Another synergy with BOTH is to remove CMA completely because
> >>>>>it makes mm core code complicated with hooking and still have a
> >>>>>problem with pinned page and eviction working set for getting
> >>>>Do you mean get_user_pages? Could you explain in details about the
> >>>>downside of CMA?
> >>>Good question.
> >>>
> >>>1. Ignore workingset.
> >>>    CMA can sweep out woring set pages in CMA area for getting contiguous
> >>>    memory.
> >>Theoritically agreed, but there's no data to prove this one.
> >CMA area is last fallback type for allocation and pages in that area would
> >be evicted out when we need contiguous memory. It means newly forked task's
> >pages would be likely in that area. newly task's pages would be fit into
> >working set category POV LRU. No?
> 
> Sorry for a silly question, what's the meaning of POV?

Nope. I'm not a native so I might be wrong. :)
http://www.abbreviations.com/POV

Thanks.

-- 
Kind regards,
Minchan Kim

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