From: Seth Jennings <sjenning@linux.vnet.ibm.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Dave Hansen <dave@linux.vnet.ibm.com>,
Joe Perches <joe@perches.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Cody P Schafer <cody@linux.vnet.ibm.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
devel@driverdev.osuosl.org
Subject: [PATCHv6 8/8] zswap: add documentation
Date: Wed, 20 Feb 2013 16:04:48 -0600 [thread overview]
Message-ID: <1361397888-14863-9-git-send-email-sjenning@linux.vnet.ibm.com> (raw)
In-Reply-To: <1361397888-14863-1-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 | 82 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 82 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..f29b82f
--- /dev/null
+++ b/Documentation/vm/zswap.txt
@@ -0,0 +1,82 @@
+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.
+
+During a page fault on a PTE that is a swap entry, frontswap calls
+the zswap load function to decompress the page into the page
+allocated by the page fault handler.
+
+Once there are no PTEs referencing a swap page stored in zswap
+(i.e. the count in the swap_map goes to 0) the swap code calls
+the zswap invalidate function, via frontswap, to free the compressed
+entry.
+
+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.
--
1.8.1.1
next prev parent reply other threads:[~2013-02-20 22:40 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-02-20 22:04 [PATCHv6 0/8] zswap: compressed swap caching Seth Jennings
2013-02-20 22:04 ` [PATCHv6 1/8] zsmalloc: add to mm/ Seth Jennings
2013-02-21 20:36 ` Cody P Schafer
2013-02-21 22:21 ` Seth Jennings
2013-02-21 22:44 ` Andrew Morton
2013-02-22 9:40 ` Joonsoo Kim
2013-02-22 17:58 ` Seth Jennings
2013-02-20 22:04 ` [PATCHv6 2/8] zsmalloc: add documentation Seth Jennings
2013-02-20 22:04 ` [PATCHv6 3/8] debugfs: add get/set for atomic types Seth Jennings
2013-02-20 22:04 ` [PATCHv6 4/8] zswap: add to mm/ Seth Jennings
2013-02-25 4:35 ` Joonsoo Kim
2013-02-25 17:21 ` Seth Jennings
2013-02-20 22:04 ` [PATCHv6 5/8] mm: break up swap_writepage() for frontswap backends Seth Jennings
2013-02-20 22:04 ` [PATCHv6 6/8] mm: allow for outstanding swap writeback accounting Seth Jennings
2013-02-20 22:04 ` [PATCHv6 7/8] zswap: add swap page writeback support Seth Jennings
2013-02-20 22:04 ` Seth Jennings [this message]
2013-02-21 15:50 ` [PATCHv6 0/8] zswap: compressed swap caching Dan Magenheimer
2013-02-21 18:25 ` Seth Jennings
2013-02-21 22:03 ` Dan Magenheimer
2013-05-01 8:06 ` Ric Mason
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