[PATCH 0/8] zswap: compressed swap caching

Seth Jennings sjenning at linux.vnet.ibm.com
Tue Dec 11 21:55:58 UTC 2012


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).

Patchset Structure:
1-4: improvements/changes to zsmalloc
5:   add atomic_t get/set to debugfs
6:   promote zsmalloc to /lib
7-8: add zswap and documentation

Targeting this for linux-next.

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.

Zswap evicts pages from compressed cache on an LRU basis to the backing
swap device when the compress pool reaches it size limit or the pool is
unable to obtain additional pages from the buddy allocator.  This
requirement had been identified in prior community discussions.

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%

Patchset is based on next-20121210

Seth Jennings (8):
  staging: zsmalloc: add gfp flags to zs_create_pool
  staging: zsmalloc: remove unsed pool name
  staging: zsmalloc: add page alloc/free callbacks
  staging: zsmalloc: make CLASS_DELTA relative to PAGE_SIZE
  debugfs: add get/set for atomic types
  zsmalloc: promote to lib/
  zswap: add to mm/
  zswap: add documentation

 Documentation/vm/zswap.txt               |   74 ++
 drivers/staging/Kconfig                  |    2 -
 drivers/staging/Makefile                 |    1 -
 drivers/staging/zcache/zcache-main.c     |    7 +-
 drivers/staging/zram/zram_drv.c          |    4 +-
 drivers/staging/zram/zram_drv.h          |    3 +-
 drivers/staging/zsmalloc/Kconfig         |   10 -
 drivers/staging/zsmalloc/Makefile        |    3 -
 drivers/staging/zsmalloc/zsmalloc-main.c | 1064 -----------------------------
 drivers/staging/zsmalloc/zsmalloc.h      |   43 --
 fs/debugfs/file.c                        |   42 ++
 include/linux/debugfs.h                  |    2 +
 include/linux/swap.h                     |    4 +
 include/linux/zsmalloc.h                 |   49 ++
 lib/Kconfig                              |   18 +
 lib/Makefile                             |    1 +
 lib/zsmalloc.c                           | 1076 +++++++++++++++++++++++++++++
 mm/Kconfig                               |   15 +
 mm/Makefile                              |    1 +
 mm/page_io.c                             |   22 +-
 mm/swap_state.c                          |    2 +-
 mm/zswap.c                               | 1077 ++++++++++++++++++++++++++++++
 22 files changed, 2383 insertions(+), 1137 deletions(-)
 create mode 100644 Documentation/vm/zswap.txt
 delete mode 100644 drivers/staging/zsmalloc/Kconfig
 delete mode 100644 drivers/staging/zsmalloc/Makefile
 delete mode 100644 drivers/staging/zsmalloc/zsmalloc-main.c
 delete mode 100644 drivers/staging/zsmalloc/zsmalloc.h
 create mode 100644 include/linux/zsmalloc.h
 create mode 100644 lib/zsmalloc.c
 create mode 100644 mm/zswap.c

-- 
1.7.9.5




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