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Date: Mon, 20 May 2013 11:26:08 -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@...1.net>, 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: [PATCHv12 4/4] zswap: add documentation This patch adds the documentation file for the zswap functionality Signed-off-by: Seth Jennings <sjenning@...ux.vnet.ibm.com> Acked-by: Rik van Riel <riel@...hat.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. -- 1.8.2.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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