Re: [PATCHv5 0/8] zswap: compressed swap caching

From: Ric Mason
Date: Fri Feb 15 2013 - 22:20:55 EST

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.


* 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

* 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)

* 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)

* 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:

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


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.


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 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.

Performance, Kernel Building:

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

* 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.

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

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at