linux-kernel - Re: [PATCHv5 2/8] zsmalloc: add documentation

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Date:	Thu, 21 Feb 2013 16:49:46 +0800
From:	Ric Mason <ric.masonn@...il.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	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@...ux.vnet.ibm.com>,
	Joe Perches <joe@...ches.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCHv5 2/8] zsmalloc: add documentation

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 <sjenning@...ux.vnet.ibm.com>
>>> ---
>>>    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

>
> Seth
>
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