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@sr71.net>, Joe Perches <joe@perches.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Cody P Schafer <cody@linux.vnet.ibm.com>,
Hugh Dickens <hughd@google.com>,
Paul Mackerras <paulus@samba.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
devel@driverdev.osuosl.org
Subject: [PATCHv13 4/4] zswap: add documentation
Date: Mon, 3 Jun 2013 15:33:05 -0500 [thread overview]
Message-ID: <1370291585-26102-5-git-send-email-sjenning@linux.vnet.ibm.com> (raw)
In-Reply-To: <1370291585-26102-1-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>
Acked-by: Rik van Riel <riel@redhat.com>
---
Documentation/vm/zswap.txt | 68 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 68 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..7e492d8
--- /dev/null
+++ b/Documentation/vm/zswap.txt
@@ -0,0 +1,68 @@
+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. zswap basically trades CPU cycles
+for potentially reduced swap I/O. This trade-off can also result in a
+significant performance improvement if reads from the compressed cache are
+faster than reads from a swap device.
+
+NOTE: Zswap is a new feature as of v3.11 and interacts heavily with memory
+reclaim. This interaction has not be fully explored on the large set of
+potential configurations and workloads that exist. For this reason, zswap
+is a work in progress and should be considered experimental.
+
+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 compressed pool reaches it size limit. 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.
+
+Zswap makes use of zbud for the managing the compressed memory pool. Each
+allocation in zbud 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 zbud
+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 one user
+controlled policies:
+* 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:[~2013-06-03 20:34 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-06-03 20:33 [PATCHv13 0/4] zswap: compressed swap caching Seth Jennings
2013-06-03 20:33 ` [PATCHv13 1/4] debugfs: add get/set for atomic types Seth Jennings
2013-06-03 20:33 ` [PATCHv13 2/4] zbud: add to mm/ Seth Jennings
2013-06-05 6:43 ` Bob Liu
2013-06-05 13:55 ` Seth Jennings
2013-06-03 20:33 ` [PATCHv13 3/4] zswap: " Seth Jennings
2013-06-17 6:20 ` Bob Liu
2013-06-17 23:02 ` Andrew Morton
2013-06-18 11:50 ` Bob Liu
2013-06-18 12:29 ` Bob Liu
2013-06-19 14:09 ` Seth Jennings
2013-06-19 14:17 ` Bob Liu
2013-06-20 2:37 ` Seth Jennings
2013-06-20 9:42 ` Bob Liu
2013-06-20 14:23 ` Seth Jennings
2013-06-20 14:35 ` Bob Liu
2013-06-21 15:20 ` Dan Magenheimer
2013-06-21 18:33 ` Konrad Rzeszutek Wilk
2013-06-03 20:33 ` Seth Jennings [this message]
2013-06-13 12:34 ` [PATCHv13 0/4] zswap: compressed swap caching Bob Liu
2013-06-13 19:06 ` Seth Jennings
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