[PATCHv13 0/4] zswap: compressed swap caching

[Seth Jennings]

This is the latest version of the zswap patchset for compressed swap caching.
This is submitted for merging into linux-next and inclusion in v3.11.

New in this Version:

Lucky 13!
Integrated feedback from Andrew and moved zbud page metadata out of the
struct page to an in-line header structure.

Useful References:

LSFMM: In-kernel memory compression
https://lwn.net/Articles/548109/

The zswap compressed swap cache
https://lwn.net/Articles/537422/

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

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.

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 zbud for the managing the compressed memory pool.
Each allocation in zbud 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 zbud 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 âcompressorâ 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.

Changelog:

v13:
* integrated feedback from Andrew
* moved zbud pool page metadata from struct page to in-line header
  contained in the page itself, avoiding potential issues with
  (broken?) code that accesses struct pages directly from the memmap
  array.

v12:
* updated Kconfig help and Documentation to indicate zswap is experimental
* remove max_compression_ratio tunable
* make zbud_pool's pages_nr lock protected and non-atomic
* don't export zbud symbols
* refactor reset_zbud_page()/__free_page() into free_zbud_page()
* make zsawp_pool_pages non-atomic
* zswap_enabled add __read_mostly
* remove type field from zswap_tree
* various comment changes

v11:
* fixup symbol collision with lib/fault-inject.c (Greg)
* rebase v3.10-rc1

v10:
* replace zsmalloc with zbud (zsmalloc to come back as option in future dev)
* lru logic moved out of zswap into allocator
* simplified and improved writeback logic
* removed memory pool and tmpage pool as part of refactoring
* Rebase to (almost) v3.10-rc1

v9:
* Fix load-during-writeback race; double lru add (for real this time)
* checkpatch and comment fixes
* Fix __swap_writepage() return value check
* Move check for max outstanding writebacks (dedup some code)
* Rebase to v3.9-rc6

v8:
* Fix load-during-writeback race; double lru add
* checkpatch fixups
* s/NOWAIT/ATOMIC for tree allocation (Dave)
* Check __swap_writepage() for error before incr outstanding write count (Rob)
* Convert pcpu compression buffer alloc from alloc_page() to kmalloc() (Dave)
* Rebase to v3.9-rc5

v7:
* Decrease zswap_stored_pages during tree cleanup (Joonsoo)
* Move zswap_entry_cache_alloc() earlier during store (Joonsoo)
* Move type field from struct zswap_entry to struct zswap_tree
* Change to swapper_space array (-rc1 change)
* s/reset_page_mapcount/page_mapcount_reset in zsmalloc (-rc1 change)
* Rebase to v3.9-rc1

v6:
* fix access-after-free regression introduced in v5
  (rb_erase() outside the lock)
* fix improper freeing of rbtree (Cody)
* fix comment typo (Ric)
* add comments about ZS_MM_WO usage and page mapping mode (Joonsoo)
* don't use page->object (Joonsoo)
* remove DEBUG (Joonsoo)
* rebase to v3.8

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

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 (4):
  debugfs: add get/set for atomic types
  zbud: add to mm/
  zswap: add to mm/
  zswap: add documentation

 Documentation/vm/zswap.txt |   68 ++++
 fs/debugfs/file.c          |   42 ++
 include/linux/debugfs.h    |    2 +
 include/linux/zbud.h       |   22 ++
 lib/fault-inject.c         |   21 -
 mm/Kconfig                 |   30 ++
 mm/Makefile                |    2 +
 mm/zbud.c                  |  526 ++++++++++++++++++++++++
 mm/zswap.c                 |  943 ++++++++++++++++++++++++++++++++++++++++++++
 9 files changed, 1635 insertions(+), 21 deletions(-)
 create mode 100644 Documentation/vm/zswap.txt
 create mode 100644 include/linux/zbud.h
 create mode 100644 mm/zbud.c
 create mode 100644 mm/zswap.c

-- 
1.7.9.5

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