Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752650Ab3EMMka (ORCPT ); Mon, 13 May 2013 08:40:30 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:44405 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750983Ab3EMMk1 (ORCPT ); Mon, 13 May 2013 08:40:27 -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: =?UTF-8?q?=5BPATCHv11=204/4=5D=20zswap=3A=20add=20documentation?= Date: Mon, 13 May 2013 07:40:03 -0500 Message-Id: <1368448803-2089-5-git-send-email-sjenning@linux.vnet.ibm.com> X-Mailer: git-send-email 1.7.9.5 In-Reply-To: <1368448803-2089-1-git-send-email-sjenning@linux.vnet.ibm.com> References: <1368448803-2089-1-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: 13051312-7182-0000-0000-000006A9309F Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4391 Lines: 94 This patch adds the documentation file for the zswap functionality Signed-off-by: Seth Jennings --- Documentation/vm/zswap.txt | 72 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 72 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..88384b3 --- /dev/null +++ b/Documentation/vm/zswap.txt @@ -0,0 +1,72 @@ +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 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 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 -- 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/