From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755459Ab3BUP57 (ORCPT ); Thu, 21 Feb 2013 10:57:59 -0500 Received: from e9.ny.us.ibm.com ([32.97.182.139]:53393 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754417Ab3BUP56 (ORCPT ); Thu, 21 Feb 2013 10:57:58 -0500 Message-ID: <5126423F.7040705@linux.vnet.ibm.com> Date: Thu, 21 Feb 2013 09:50:23 -0600 From: Seth Jennings User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130106 Thunderbird/17.0.2 MIME-Version: 1.0 To: Ric Mason CC: Andrew Morton , 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 , linux-mm@kvack.org, linux-kernel@vger.kernel.org, devel@driverdev.osuosl.org Subject: Re: [PATCHv5 2/8] zsmalloc: add documentation References: <1360780731-11708-1-git-send-email-sjenning@linux.vnet.ibm.com> <1360780731-11708-3-git-send-email-sjenning@linux.vnet.ibm.com> <511F254D.2010909@gmail.com> <51227DF4.9020900@linux.vnet.ibm.com> <5125DFAA.4050706@gmail.com> In-Reply-To: <5125DFAA.4050706@gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13022115-7182-0000-0000-0000056D021A Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/21/2013 02:49 AM, Ric Mason wrote: > On 02/19/2013 03:16 AM, Seth Jennings wrote: >> On 02/16/2013 12:21 AM, Ric Mason wrote: >>> On 02/14/2013 02:38 AM, Seth Jennings wrote: >>>> This patch adds a documentation file for zsmalloc at >>>> Documentation/vm/zsmalloc.txt >>>> >>>> Signed-off-by: Seth Jennings >>>> --- >>>> Documentation/vm/zsmalloc.txt | 68 >>>> +++++++++++++++++++++++++++++++++++++++++ >>>> 1 file changed, 68 insertions(+) >>>> create mode 100644 Documentation/vm/zsmalloc.txt >>>> >>>> diff --git a/Documentation/vm/zsmalloc.txt >>>> b/Documentation/vm/zsmalloc.txt >>>> new file mode 100644 >>>> index 0000000..85aa617 >>>> --- /dev/null >>>> +++ b/Documentation/vm/zsmalloc.txt >>>> @@ -0,0 +1,68 @@ >>>> +zsmalloc Memory Allocator >>>> + >>>> +Overview >>>> + >>>> +zmalloc a new slab-based memory allocator, >>>> +zsmalloc, for storing compressed pages. It is designed for >>>> +low fragmentation and high allocation success rate on >>>> +large object, but <= PAGE_SIZE allocations. >>>> + >>>> +zsmalloc differs from the kernel slab allocator in two primary >>>> +ways to achieve these design goals. >>>> + >>>> +zsmalloc never requires high order page allocations to back >>>> +slabs, or "size classes" in zsmalloc terms. Instead it allows >>>> +multiple single-order pages to be stitched together into a >>>> +"zspage" which backs the slab. This allows for higher allocation >>>> +success rate under memory pressure. >>>> + >>>> +Also, zsmalloc allows objects to span page boundaries within the >>>> +zspage. This allows for lower fragmentation than could be had >>>> +with the kernel slab allocator for objects between PAGE_SIZE/2 >>>> +and PAGE_SIZE. With the kernel slab allocator, if a page compresses >>>> +to 60% of it original size, the memory savings gained through >>>> +compression is lost in fragmentation because another object of >>>> +the same size can't be stored in the leftover space. >>>> + >>>> +This ability to span pages results in zsmalloc allocations not being >>>> +directly addressable by the user. The user is given an >>>> +non-dereferencable handle in response to an allocation request. >>>> +That handle must be mapped, using zs_map_object(), which returns >>>> +a pointer to the mapped region that can be used. The mapping is >>>> +necessary since the object data may reside in two different >>>> +noncontigious pages. >>> Do you mean the reason of to use a zsmalloc object must map after >>> malloc is object data maybe reside in two different nocontiguous pages? >> Yes, that is one reason for the mapping. The other reason (more of an >> added bonus) is below. >> >>>> + >>>> +For 32-bit systems, zsmalloc has the added benefit of being >>>> +able to back slabs with HIGHMEM pages, something not possible >>> What's the meaning of "back slabs with HIGHMEM pages"? >> By HIGHMEM, I'm referring to the HIGHMEM memory zone on 32-bit systems >> with larger that 1GB (actually a little less) of RAM. The upper 3GB >> of the 4GB address space, depending on kernel build options, is not >> directly addressable by the kernel, but can be mapped into the kernel >> address space with functions like kmap() or kmap_atomic(). >> >> These pages can't be used by slab/slub because they are not >> continuously mapped into the kernel address space. However, since >> zsmalloc requires a mapping anyway to handle objects that span >> non-contiguous page boundaries, we do the kernel mapping as part of >> the process. >> >> So zspages, the conceptual slab in zsmalloc backed by single-order >> pages can include pages from the HIGHMEM zone as well. > > Thanks for your clarify, > http://lwn.net/Articles/537422/, your article about zswap in lwn. > "Additionally, the kernel slab allocator does not allow objects that > are less > than a page in size to span a page boundary. This means that if an > object is > PAGE_SIZE/2 + 1 bytes in size, it effectively use an entire page, > resulting in > ~50% waste. Hense there are *no kmalloc() cache size* between > PAGE_SIZE/2 and > PAGE_SIZE." > Are your sure? It seems that kmalloc cache support big size, your can > check in > include/linux/kmalloc_sizes.h Yes, kmalloc can allocate large objects > PAGE_SIZE, but there are no cache sizes _between_ PAGE_SIZE/2 and PAGE_SIZE. For example, on a system with 4k pages, there are no caches between kmalloc-2048 and kmalloc-4096. Seth