Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765100Ab3DDWLb (ORCPT ); Thu, 4 Apr 2013 18:11:31 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:54881 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1765070Ab3DDWL0 (ORCPT ); Thu, 4 Apr 2013 18:11:26 -0400 From: Seth Jennings To: Andrew Morton Cc: Seth Jennings , Greg Kroah-Hartman , Nitin Gupta , Minchan Kim , Konrad Rzeszutek Wilk , Dan Magenheimer , Robert Jennings , Jenifer Hopper , Mel Gorman , Johannes Weiner , Rik van Riel , Larry Woodman , Benjamin Herrenschmidt , Dave Hansen , Joe Perches , Joonsoo Kim , Cody P Schafer , Hugh Dickens , Paul Mackerras , linux-mm@kvack.org, linux-kernel@vger.kernel.org, devel@driverdev.osuosl.org Subject: [PATCHv8 8/8] zswap: add documentation Date: Thu, 4 Apr 2013 17:10:46 -0500 Message-Id: <1365113446-25647-9-git-send-email-sjenning@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.2 In-Reply-To: <1365113446-25647-1-git-send-email-sjenning@linux.vnet.ibm.com> References: <1365113446-25647-1-git-send-email-sjenning@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13040422-2876-0000-0000-0000071B7B66 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4675 Lines: 104 This patch adds the documentation file for the zswap functionality Signed-off-by: Seth Jennings --- 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. -- 1.8.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/