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Message-Id: <1365617940-21623-9-git-send-email-sjenning@linux.vnet.ibm.com>
Date:	Wed, 10 Apr 2013 13:19:00 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Joe Perches <joe@...ches.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>,
	Hugh Dickens <hughd@...gle.com>,
	Paul Mackerras <paulus@...ba.org>,
	Heesub Shin <heesub.shin@...sung.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: [PATCHv9 8/8] zswap: add documentation

This patch adds the documentation file for the zswap functionality

Signed-off-by: Seth Jennings <sjenning@...ux.vnet.ibm.com>
---
 Documentation/vm/zsmalloc.txt |  2 +-
 Documentation/vm/zswap.txt    | 82 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 83 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/vm/zswap.txt

diff --git a/Documentation/vm/zsmalloc.txt b/Documentation/vm/zsmalloc.txt
index 85aa617..4133ade 100644
--- a/Documentation/vm/zsmalloc.txt
+++ b/Documentation/vm/zsmalloc.txt
@@ -65,4 +65,4 @@ zs_unmap_object(pool, handle);
 zs_free(pool, handle);
 
 /* destroy the pool */
-zs_destroy_pool(pool); 
+zs_destroy_pool(pool);
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.
-- 
1.8.2.1

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