From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx151.postini.com [74.125.245.151]) by kanga.kvack.org (Postfix) with SMTP id 19C966B0074 for ; Wed, 8 May 2013 18:38:08 -0400 (EDT) Received: from /spool/local by e9.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 8 May 2013 18:38:07 -0400 Received: from d01relay04.pok.ibm.com (d01relay04.pok.ibm.com [9.56.227.236]) by d01dlp02.pok.ibm.com (Postfix) with ESMTP id 1834A6E803F for ; Wed, 8 May 2013 18:38:01 -0400 (EDT) Received: from d01av01.pok.ibm.com (d01av01.pok.ibm.com [9.56.224.215]) by d01relay04.pok.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id r48Mc4nX315550 for ; Wed, 8 May 2013 18:38:04 -0400 Received: from d01av01.pok.ibm.com (loopback [127.0.0.1]) by d01av01.pok.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id r48Mc3ik031485 for ; Wed, 8 May 2013 18:38:04 -0400 From: Seth Jennings Subject: [PATCHv10 4/4] zswap: add documentation Date: Wed, 8 May 2013 17:37:41 -0500 Message-Id: <1368052661-27143-5-git-send-email-sjenning@linux.vnet.ibm.com> In-Reply-To: <1368052661-27143-1-git-send-email-sjenning@linux.vnet.ibm.com> References: <1368052661-27143-1-git-send-email-sjenning@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: 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 , Heesub Shin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, devel@driverdev.osuosl.org 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.A 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.A 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 +A A A performance impact of swapping. +* Overcommitted guests that share a common I/O resource can +A A A dramatically reduce their swap I/O pressure, avoiding heavy handed I/O + throttling by the hypervisor.A 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 +A A A 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 +a??compressora?? 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. -- 1.8.2.2 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org