Message-ID: <511EFB05.3020700@gmail.com>
Date:	Sat, 16 Feb 2013 11:20:37 +0800
From:	Ric Mason <ric.masonn@...il.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Joe Perches <joe@...ches.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCHv5 0/8] zswap: compressed swap caching

On 02/14/2013 02:38 AM, Seth Jennings wrote:
> Lots of changes this time around.  Hopefully I collected and acted on
> all the feedback.  I apologize ahead of time if I missed something.
> Please let me know if I did.
>
> Changelog:
>
> v5:
> * zsmalloc patch converted from promotion to "new code" (for review only,
>    see note in [1/8])
> * promote zsmalloc to mm/ instead of /lib
> * add more documentation everywhere
> * convert USE_PGTABLE_MAPPING to kconfig option, thanks to Minchan
> * s/flush/writeback/
> * #define pr_fmt() for formatting messages (Joe)
> * checkpatch fixups
> * lots of changes suggested Minchan
>
> v4:
> * Added Acks (Minchan)
> * Separated flushing functionality into standalone patch
>    for easier review (Minchan)
> * fix comment on zswap enabled attribute (Minchan)
> * add TODO for dynamic mempool size (Minchan)
> * and check for NULL in zswap_free_page() (Minchan)
> * add missing zs_free() in error path (Minchan)
> * TODO: add comments for flushing/refcounting (Minchan)
>
> v3:
> * Dropped the zsmalloc patches from the set, except the promotion patch
>    which has be converted to a rename patch (vs full diff).  The dropped
>    patches have been Acked and are going into Greg's staging tree soon.
> * Separated [PATCHv2 7/9] into two patches since it makes changes for two
>    different reasons (Minchan)
> * Moved ZSWAP_MAX_OUTSTANDING_FLUSHES near the top in zswap.c (Rik)
> * Rebase to v3.8-rc5. linux-next is a little volatile with the
>    swapper_space per type changes which will effect this patchset.
> * TODO: Move some stats from debugfs to sysfs. Which ones? (Rik)
>
> v2:
> * Rename zswap_fs_* functions to zswap_frontswap_* to avoid
>    confusion with "filesystem"
> * Add comment about what the tree lock protects
> * Remove "#if 0" code (should have been done before)
> * Break out changes to existing swap code into separate patch
> * Fix blank line EOF warning on documentation file
> * Rebase to next-20130107
>
> Zswap Overview:
>
> Zswap is a lightweight compressed cache for swap pages. It takes
> pages that are in the process of being swapped out and attempts to
> compress them into a dynamically allocated RAM-based memory pool.
> If this process is successful, the writeback to the swap device is
> deferred and, in many cases, avoided completely.  This results in
> a significant I/O reduction and performance gains for systems that
> are swapping.
>
> The results of a kernel building benchmark indicate a
> runtime reduction of 53% and an I/O reduction 76% with zswap vs normal
> swapping with a kernel build under heavy memory pressure (see
> Performance section for more).
>
> Some addition performance metrics regarding the performance
> improvements and I/O reductions that can be achieved using zswap as
> measured by SPECjbb are provided here:
>
> http://ibm.co/VCgHvM

I see this link.  You mentioned that "When a user enables zswap and the 
hardware accelerator, zswap simply passes the pages to be compressed or 
decompressed off to the accelerator instead of performing the work in 
software". Then how can user enable hardware accelerator, there are 
option in UEFI or ... ?

> These results include runs on x86 and new results on Power7+ with
> hardware compression acceleration.
>
> Of particular note is that zswap is able to evict pages from the compressed
> cache, on an LRU basis, to the backing swap device when the compressed pool
> reaches it size limit or the pool is unable to obtain additional pages
> from the buddy allocator.  This eviction functionality had been identified
> as a requirement in prior community discussions.
>
> Patchset Structure:
> 1-2: add zsmalloc and documentation
> 3:   add atomic_t get/set to debugfs
> 4:   add basic zswap functionality
> 4,5: changes to existing swap code for zswap
> 6,7: add zswap writeback support
> 8:   add zswap documentation
>
> Rationale:
>
> Zswap provides compressed swap caching that basically trades CPU cycles
> for reduced swap I/O.  This trade-off can result in a significant
> performance improvement as reads to/writes from to the compressed
> cache almost always faster that reading from a swap device
> which incurs the latency of an asynchronous block I/O read.
>
> Some potential benefits:
> * Desktop/laptop users with limited RAM capacities can mitigate the
>      performance impact of swapping.
> * Overcommitted guests that share a common I/O resource can
>      dramatically reduce their swap I/O pressure, avoiding heavy
>      handed I/O throttling by the hypervisor.  This allows more work
>      to get done with less impact to the guest workload and guests
>      sharing the I/O subsystem
> * Users with SSDs as swap devices can extend the life of the device by
>      drastically reducing life-shortening writes.
>
> Compressed swap is also provided in zcache, along with page cache
> compression and RAM clustering through RAMSter. Zswap seeks to deliver
> the benefit of swap  compression to users in a discrete function.
> This design decision is akin to Unix design philosophy of doing one
> thing well, it leaves file cache compression and other features
> for separate code.
>
> Design:
>
> Zswap receives pages for compression through the Frontswap API and
> is able to evict pages from its own compressed pool on an LRU basis
> and write them back to the backing swap device in the case that the
> compressed pool is full or unable to secure additional pages from
> the buddy allocator.
>
> Zswap makes use of zsmalloc for the managing the compressed memory
> pool.  This is because zsmalloc is specifically designed to minimize
> fragmentation on large (> PAGE_SIZE/2) allocation sizes.  Each
> allocation in zsmalloc is not directly accessible by address.
> Rather, a handle is return by the allocation routine and that handle
> must be mapped before being accessed.  The compressed memory pool grows
> on demand and shrinks as compressed pages are freed.  The pool is
> not preallocated.
>
> When a swap page is passed from frontswap to zswap, zswap maintains
> a mapping of the swap entry, a combination of the swap type and swap
> offset, to the zsmalloc handle that references that compressed swap
> page.  This mapping is achieved with a red-black tree per swap type.
> The swap offset is the search key for the tree nodes.
>
> Zswap seeks to be simple in its policies.  Sysfs attributes allow for
> two user controlled policies:
> * max_compression_ratio - Maximum compression ratio, as as percentage,
>      for an acceptable compressed page. Any page that does not compress
>      by at least this ratio will be rejected.
> * max_pool_percent - The maximum percentage of memory that the compressed
>      pool can occupy.
>
> To enabled zswap, the "enabled" attribute must be set to 1 at boot time.
>
> Zswap allows the compressor to be selected at kernel boot time by
> setting the “compressor” attribute.  The default compressor is lzo.
>
> A debugfs interface is provided for various statistic about pool size,
> number of pages stored, and various counters for the reasons pages
> are rejected.
>
> Performance, Kernel Building:
>
> Setup
> ========
> Gentoo w/ kernel v3.7-rc7
> Quad-core i5-2500 @ 3.3GHz
> 512MB DDR3 1600MHz (limited with mem=512m on boot)
> Filesystem and swap on 80GB HDD (about 58MB/s with hdparm -t)
> majflt are major page faults reported by the time command
> pswpin/out is the delta of pswpin/out from /proc/vmstat before and after
> the make -jN
>
> Summary
> ========
> * Zswap reduces I/O and improves performance at all swap pressure levels.
>
> * Under heavy swaping at 24 threads, zswap reduced I/O by 76%, saving
>    over 1.5GB of I/O, and cut runtime in half.
>
> Details
> ========
> I/O (in pages)
> 	base				zswap				change	change
> N	pswpin	pswpout	majflt	I/O sum	pswpin	pswpout	majflt	I/O sum	%I/O	MB
> 8	1	335	291	627	0	0	249	249	-60%	1
> 12	3688	14315	5290	23293	123	860	5954	6937	-70%	64
> 16	12711	46179	16803	75693	2936	7390	46092	56418	-25%	75
> 20	42178	133781	49898	225857	9460	28382	92951	130793	-42%	371
> 24	96079	357280	105242	558601	7719	18484	109309	135512	-76%	1653
>
> Runtime (in seconds)
> N	base	zswap	%change
> 8	107	107	0%
> 12	128	110	-14%
> 16	191	179	-6%
> 20	371	240	-35%
> 24	570	267	-53%
>
> %CPU utilization (out of 400% on 4 cpus)
> N	base	zswap	%change
> 8	317	319	1%
> 12	267	311	16%
> 16	179	191	7%
> 20	94	143	52%
> 24	60	128	113%
>
> Seth Jennings (8):
>    zsmalloc: add to mm/
>    zsmalloc: add documentation
>    debugfs: add get/set for atomic types
>    zswap: add to mm/
>    mm: break up swap_writepage() for frontswap backends
>    mm: allow for outstanding swap writeback accounting
>    zswap: add swap page writeback support
>    zswap: add documentation
>
>   Documentation/vm/zsmalloc.txt |   68 +++
>   Documentation/vm/zswap.txt    |   82 +++
>   fs/debugfs/file.c             |   42 ++
>   include/linux/debugfs.h       |    2 +
>   include/linux/swap.h          |    4 +
>   include/linux/zsmalloc.h      |   49 ++
>   mm/Kconfig                    |   39 ++
>   mm/Makefile                   |    2 +
>   mm/page_io.c                  |   22 +-
>   mm/swap_state.c               |    2 +-
>   mm/zsmalloc.c                 | 1124 ++++++++++++++++++++++++++++++++++++++++
>   mm/zswap.c                    | 1148 +++++++++++++++++++++++++++++++++++++++++
>   12 files changed, 2578 insertions(+), 6 deletions(-)
>   create mode 100644 Documentation/vm/zsmalloc.txt
>   create mode 100644 Documentation/vm/zswap.txt
>   create mode 100644 include/linux/zsmalloc.h
>   create mode 100644 mm/zsmalloc.c
>   create mode 100644 mm/zswap.c
>

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