RE: [PATCHv5 2/8] zsmalloc: add documentation
From: Dan Magenheimer
Date: Thu Feb 21 2013 - 11:24:04 EST
> From: Seth Jennings [mailto:sjenning@xxxxxxxxxxxxxxxxxx]
> Subject: Re: [PATCHv5 2/8] zsmalloc: add documentation
> 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 <sjenning@xxxxxxxxxxxxxxxxxx>
> >>>> ---
> >>>> 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
Important and left unsaid here is that, in many workloads, the
distribution of compressed pages ("zpages") will have as many
as half or more with compressed size ("zsize") between PAGE_SIZE/2
and PAGE_SIZE. And, in many workloads, the majority of values for
zsize will be much closer to PAGE_SIZE/2 than PAGE_SIZE, which will
result in a great deal of wasted space if slab were used.
And, also very important, kmalloc requires page allocations with
"order > 0" (2**n contiguous pages) to deal with "big size objects".
In-kernel compression would need many of these and they are difficult
(often impossible) to allocate when the system is under memory
As a result, various other allocators have been written, first
xvmalloc, then zbud, then zsmalloc. Each of these depend only
on order==0 page allocations and each has ways of dealing with
high quantities of zpages with PAGE_SIZE/2 < zsize < PAGE_SIZE.
Hope that helps!
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/