From: Seth Jennings <sjenning@linux.vnet.ibm.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Andrew Morton <akpm@linux-foundation.org>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>,
Nitin Gupta <ngupta@vflare.org>, Minchan Kim <minchan@kernel.org>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
Dan Magenheimer <dan.magenheimer@oracle.com>,
Robert Jennings <rcj@linux.vnet.ibm.com>,
Jenifer Hopper <jhopper@us.ibm.com>, Mel Gorman <mgorman@suse.de>,
Johannes Weiner <jweiner@redhat.com>,
Rik van Riel <riel@redhat.com>,
Larry Woodman <lwoodman@redhat.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
devel@driverdev.osuosl.org
Subject: [PATCHv2 9/9] zswap: add documentation
Date: Mon, 7 Jan 2013 14:24:40 -0600 [thread overview]
Message-ID: <1357590280-31535-10-git-send-email-sjenning@linux.vnet.ibm.com> (raw)
In-Reply-To: <1357590280-31535-1-git-send-email-sjenning@linux.vnet.ibm.com>
This patch adds the documentation file for the zswap functionality
Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
---
Documentation/vm/zswap.txt | 73 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 73 insertions(+)
create mode 100644 Documentation/vm/zswap.txt
diff --git a/Documentation/vm/zswap.txt b/Documentation/vm/zswap.txt
new file mode 100644
index 0000000..5d00ce9
--- /dev/null
+++ b/Documentation/vm/zswap.txt
@@ -0,0 +1,73 @@
+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.
+
+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.
+
+To enabled zswap, the "enabled" attribute must be set to 1 at boot time.
+e.g. zswap.enabled=1
+
+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.
+
+Zswap allows the compressor to be selected at kernel boot time by
+setting the “compressor” attribute. The default compressor is lzo.
+e.g. zswap.compressor=deflate
+
+A debugfs interface is provided for various statistic about pool size,
+number of pages stored, and various counters for the reasons pages
+are rejected.
--
1.7.9.5
next prev parent reply other threads:[~2013-01-07 20:25 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-01-07 20:24 [PATCHv2 0/9] zswap: compressed swap caching Seth Jennings
2013-01-07 20:24 ` [PATCHv2 1/9] staging: zsmalloc: add gfp flags to zs_create_pool Seth Jennings
2013-01-25 0:08 ` Nitin Gupta
2013-01-25 1:33 ` Minchan Kim
2013-01-25 15:07 ` Seth Jennings
2013-01-25 15:56 ` Dan Magenheimer
2013-01-28 2:59 ` Minchan Kim
2013-01-30 16:11 ` Konrad Rzeszutek Wilk
2013-01-31 5:21 ` Minchan Kim
2013-01-25 21:26 ` Rik van Riel
2013-01-07 20:24 ` [PATCHv2 2/9] staging: zsmalloc: remove unsed pool name Seth Jennings
2013-01-25 0:09 ` Nitin Gupta
2013-01-25 21:50 ` Rik van Riel
2013-01-07 20:24 ` [PATCHv2 3/9] staging: zsmalloc: add page alloc/free callbacks Seth Jennings
2013-01-25 0:11 ` Nitin Gupta
2013-01-25 21:55 ` Rik van Riel
2013-01-07 20:24 ` [PATCHv2 4/9] staging: zsmalloc: make CLASS_DELTA relative to PAGE_SIZE Seth Jennings
2013-01-25 0:17 ` Nitin Gupta
2013-01-25 16:38 ` Seth Jennings
2013-01-07 20:24 ` [PATCHv2 5/9] debugfs: add get/set for atomic types Seth Jennings
2013-01-07 20:32 ` Greg Kroah-Hartman
2013-01-07 20:41 ` Seth Jennings
2013-01-25 16:45 ` Seth Jennings
2013-01-25 21:35 ` Greg Kroah-Hartman
2013-01-07 20:24 ` [PATCHv2 6/9] zsmalloc: promote to lib/ Seth Jennings
2013-01-28 4:01 ` Minchan Kim
2013-01-28 4:32 ` Minchan Kim
2013-01-28 17:41 ` Seth Jennings
2013-01-07 20:24 ` [PATCHv2 7/9] mm: break up swap_writepage() for frontswap backends Seth Jennings
2013-01-28 4:22 ` Minchan Kim
2013-01-28 17:26 ` Seth Jennings
2013-01-28 23:46 ` Minchan Kim
2013-01-07 20:24 ` [PATCHv2 8/9] zswap: add to mm/ Seth Jennings
2013-01-08 17:15 ` Dave Hansen
2013-01-08 17:54 ` Dan Magenheimer
2013-01-25 22:44 ` Rik van Riel
2013-01-25 23:15 ` Dan Magenheimer
2013-01-28 15:27 ` Seth Jennings
2013-01-29 10:21 ` Lord Glauber Costa of Sealand
2013-02-07 16:13 ` Seth Jennings
2013-02-11 19:13 ` Dan Magenheimer
2013-01-07 20:24 ` Seth Jennings [this message]
2013-01-22 18:10 ` [PATCHv2 0/9] zswap: compressed swap caching Seth Jennings
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