Re: zcache+zram working together?

[Minchan Kim <minchan@kernel.org>]

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
contiguous memory.

If we can replace CMA with zcache/zram, it would be very good.

1. Remove many core code hooking
2. zcache/zram will have eviction pages of LRU order so
   discarding of them would be much sense when user want to get
   contiguous memory space.

> Dan
> 
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-- 
Kind regards,
Minchan Kim

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