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@xxxxxxxxxx> 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?
2. No guarantee of contigous memory area
As I metioned, get_user_pages could pin the page so ends up failing
migration.
Right it's working item now, we have to guarantee these pages can't
allocate from CMA area.
Good to hear. CMA's goal is to guarantee it.
If it can't, there is no point to use it. FYI, memory-hotplug people have
same problem and have tried to solve it and I wanted they should solve
CMA problem by their solution, too but not sure they do.
I'm looking forwading to seeing your elegant works.
3. Latency
CMA reclaims all pages in CMA area when we need it. It means sometime
we should write out dirty pages so it could make big overhead POV latency.
Even, unmapping of all pages from pte of all processes isn't trivial.
It's trade off between requirement and performance. If feature is more
important and need more memory, it can accept it
It depends on usecase and as you already know, many people want to use
CMA with small latency if possible. If CMA can't meet their latency
requirement, they might use reserved memory rather than CMA or use CMA
with some harmful jobs(ex, sync + drop_cache).
If kernel can provide better solution, they can avoid such things.
4. Adding many hooks in MM code. - Personally, I really hate it.
But there are cases to use CMA. e.g., DRM playback.
We have to guarantee the physical contiguous memory for TrustZone
solution at ARM.
Without reseverd memory concept. there's no way to get physical
congituous memory execpt CMA.
I don't get it.
I meant I don't like CONFIG_CMA hook under mm/.
Thank you,
Kyungmin Park
--
Kind regards,
Minchan Kim
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