Re: [PATCHv5 2/8] zsmalloc: add documentation

[Ric Mason]

On 02/21/2013 11:50 PM, Seth Jennings wrote:
> 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
> kmalloc-4096.

Since slub cache can merge, is it the root reason?

> Seth
>
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